Younes Kaboul could be out for two months with a hamstring problem, one he picked up in the 3-1 defeat to Chelsea two weeks ago.
The Sunderland defender has been key for his side this season since signing from Tottenham Hotspur in the summer, with his manager, Sam Allardyce, recognising this when he was unavailable through illness in the 1-0 defeat to Watford.
However, he will have to find a solution to the Frenchman's absence as he is going to be without him for some time now, with the boss confirming they have "lost" him "for six to eight weeks" ahead of Wednesday's clash with Liverpool.
Back to a four man defence
Kaboul's absence will lead to a switch in formation, that's for sure. Allardyce has tried to play the 3-5-2 set-up without his preferred back three of Kaboul, John O'Shea and Sebastian Coates several times this season, but never successfully.
On Wednesday, he will be without both the Frenchman and O'Shea through injury, which should signal the start of a temporary return to a back four.
This problem can be rectified in January as the transfer window opens, allowing Allardyce the opportunity to bring in some more centre-backs, but for now it will be a 4-5-1 system for the team it seems.
Defensive woes
However, the options to partner Coates this week are not great with O'Shea having limped off against Manchester City last time out.
Wes Brown should return to play alongside the Uruguayan, but he "has only just come back from a bit of an injury," Allardyce said, whilst he has obviously not "played that much" either.
Tom Beadling is another option, the youngster now training regularly with the first team since returning from his own injury a few weeks ago, whilst Jack Rodwell could drop into a centre-back role he started his career in, returning from suspension this week.
Right-back Billy Jones has also played in the heart of the back line this season, but the fact that these are Allardyce's only options show how thin Sunderland are in defence.
Whilst the former Newcastle United boss searches for a temporary fix on Wednesday, the situation is a clear reminder that defensive recruits are needed in January.