Players such as Daniel Ayala, Adam Clayton and Gaston Ramirez were recieving plaudits from fans, journalists and pundits alike.
Out of the 47 players signed by the former Real Madrid and Athletic Bilbao defender, only a handful have not been successful on Teesside.
Below, however, are the three players who would be Aitor Karanka's biggest transfer regrets.
To some Boro fans, this name might not mean a lot.
This is because the Dutch international Tiendalli only made two appearances for Middlesbrough. A two-nil defeat against Watford at Vicarage Road and a nil-nil draw with Brighton to end the 2014/15 season which saw Boro take on Brentford in the play offs.
Tiendalli looked very shaky in both matches, and he did not live up to any expectations anyone may have had from a Premier League player dropping down a league.
Possibly Karanka's most forgettable signing, Tiendalli, did not cost Middlesbrough in any large way, however.
The additional five points that Boro might have gained from the two games would have seen the Teessiders promoted, but Tiendalli's absence would surely not have affected those matches to such an extent.
A possible surprise inclusion here, as the Chelsea loanee won Championship player of the year, during his season in the North East.
The regret Aitor Karanka might have with Bamford, however, is that he failed to keep him on Teesside for a longer period.
The former Nottingham Forest forward was one of England's hottest prospects when he returned to London after scoring 17 times for Boro, but has failed to recreate the form he found in the 2014/15 campaign.
Bamford himself may regret not returning to Teesside. Now at Burnley after not scoring since April 14th 2015 in Boro's win against Wolves, the Chelsea striker will hope to get back on the goal trail even if that means scoring against Boro.
Middlesbrough, however, did not need him to get promoted as they made their return to the Premier League last season, after a seven year absence.
The Teessiders now have a superior strike force and Karanka hopes Jordan Rhodes and Alvaro Negredo can score the goals to keep Boro up.
Carlos De Pena
To clarify, De Pena is still a young player and has room to improve. A loan move away from the Riverside may help to Uruguayan reignite his career on Teesside.
After a £2.5 million move from Uruguayan club Nacional on deadline day of the summer 2015 window, Middlesbrough fans hoped that De Pena would bring flare to their side's left flank.
This never really came to be, however, and De Pena has only featured six times in England.
Despite the fact that he is just 24 and has room to improve, Steve Gibson if not Aitor Karanka, may regret paying such a fee for someone who has, so far, failed to live up to standards.