Sunderland's season last year was marked by a number of laughable defensive performances, which contributed heavily to their concerning 16th place finish.

The Black Cats ended the campaign just two places and three points above the relegation places, with the fans having to endure another stressful year in the stands as they watched their team get far too involved in the scrap for survival.

After signing a one-year deal with the club this summer, manager Dick Advocaat is now tasked with rectifying these defensive problems in particular as he bids to establish Sunderland as a strong Premier League side again, with them having struggled to do since their most recent return to the top flight in 2007.

It is important for him to leave Wearside off the back of a solid season, putting the team in good stead for the next campaign as the club search for the Dutchman's successor. He has made a number of signings already this summer, with three of his four recruits being defenders, but is this enough to improve the back line?

Signings add depth and competition

Advocaat has spent £7 million on his three defensive signings this summer, securing the services of right-back Adam Matthews and centre-backs Younes Kaboul and Sebastian Coates. Each of these players have European experience and international caps to their name, whilst the latter two have plenty of experience in the Premier League and the former in a title-winning team, having spent several years absorbing the winning mentality at Celtic.

As well as this though, the three players each have points to prove. Coates was unable to make the jump from the youth set-up to the first team at Liverpool, leading to his sale; Kaboul fell out-of-favour last season with Tottenham Hotspur; whilst Matthews is hoping to establish himself in the Premier League and reclaim his place in the Wales national team.

After Santiago Vergini was loaned out to Getafe for the coming season, Matthews provides Billy Jones with competition at right-back which will ensure both players are pushing themselves as hard as possible for a starting berth. Matthews is keen to play in the Premier League, having spent time with Cardiff City in the Championship previously in his career but never in the top flight, and so he will work his hardest to earn as many minutes as he can, which will, in turn, bring the best out of Jones too.

In the centre of defence, Sunderland now have four options, with Coates and Kaboul adding to the presence of former-Manchester United duo John O'Shea and Wes Brown. Both of these two put on some poor performances last season that received plenty of criticism, but Sunderland lacked players in the position to replace them and thus had to keep their faith in the two veterans. This year, however, each player will have to be playing at their very best to get a game, as there are plenty of options for Advocaat to pick from.

Still lacking strength at left-back

Despite the three new arrivals, one position in defence that still has not been strengthened is left-back. Patrick van Aanholt has established himself in the Sunderland side since joining from Chelsea last summer, but his attacking, wideman-like approach to games often leaves the Black Cats exposed on their left hand side.

There's no question that the former Blue adds to the team's offensive threat, but, even with his electric pace, he isn't always able to track back after helping out going forward. When he does defend, van Aanholt is very effective. He is strong, quick and surprisingly not over-zealous despite his young age of 24.

However, if Sunderland are to improve at the back, the Dutchman is either going to have to focus more on defending than attacking and become more disciplined, which could stifle the team's threat on the left, which promises to be strong this season following the arrival of Jeremain Lens, or van Aanholt needs a competitor for his place to push him to put in that extra work tracking back, and more work off the pitch furthering his fitness to allow him to do this too.

Both Matthews and Jones are capable of playing left-back in the event of an injury or suspension to their teammate, but this would not be ideal for Advocaat and his side given that neither are naturally left-sided players or left-footed. Thus, another player in van Aanholt's position would certainly be a useful addition to the squad this summer.

Defensive midfielder could be recruited instead

As aforementioned, forcing van Aanholt to be more disciplined in his play is something that could hinder Sunderland going forward, as the Dutchman is a menace when he gets going on the left. He and Lens, compatriots with a good relationship, look set to form a formidable partnership down that flank this year, and this is something that Advocaat will not want to stifle.

Therefore, an alternative way to deal with this risk presented by the 24-year-old's attacking mindset is to recruit a defensive midfielder, one who can drop back and sit effectively when his full-back, or full-backs, bomb forward.

It is seen frequently in the best teams - the right-back and left-back are present in the attacking third, whilst the holding midfield drops into defence to form a back three with the two remaining centre-backs. This is something Sunderland need to work on in their tactics to ensure more protection for their goalkeeper, but they also need the right personnel to do so.

Lee Cattermole is a fantastic player, a real fighter and a hard-worker. However, he is best operating as a box-to-box midfielder, not as a defensive midfielder, as he commits too many fouls and is also a great asset going forward with clever vision and wicked strike on him.

Cattermole was often Sunderland's deepest midfield player last season though, and his weaknesses were exploited in the position, meaning his side were exploited at the back. With Advocaat looking to offload Liam Bridcutt, a natural holding midfielder, the team needs someone - not Cattermole -  to sit in this position this season if they want to improve defensively.

The Black Cats are reportedly close to finalising a move for Rubin Kazan's Yann M'Vila, and he would be a terrific signing for the club given his experience and high pedigree. M'Vila is defensive midfielder and his physical presence and ability on the ball would go a long way in helping clean up the mess Sunderland are in at the back.

Finishing touches on improvement job needed

There is no question that Sunderland's defence this season will be much improved from that which conceded 53 goals in 38 games last season.

The added depth and competition at the back will prove to be a huge help as the Black Cats look to go further than just securing survival this season, with rotation and cover for injuries also a bonus that will come from Advocaat's strong summer business.

Coates is already settled into the team after spending last season on loan, Kaboul doesn't need to worry about settling into the league having spent the majority of his career in the English top flight, whilst Matthews should adjust quickly having played at the top level and also in England before.

However, there is still a problem on the left-hand side that can only be solved by bringing in another left-back or a defensive midfielder to cover when van Aanholt bombs forward. New signing Lens tracks back most of the time, but laziness can occur and may do when most unwanted and allow Sunderland to be completely exposed on the left.

Completing a move for M'Vila, or another defensive midfielder, would certainly apply most of the finishing touches to Advocaat's renovations in defence so far, though another may be needed as a back-up if Bridcutt is to leave the club to prevent Cattermole from having to drop in due to injuries or suspensions.

The defensive problems at Sunderland have been looked at, taken on board and certainly improved, but the job is not done yet.