Sunderland will know within the next 48-hours the severity of Paddy McNair's groin injury.
Boss Chris Coleman is sweating on the results as he believes McNair is a huge player in the centre of the Sunderland midfield when fit.
The Sunderland midfielder was subbed three minutes before half-time after injuring his groin, Jonny Williams coming on to replace him just before the break.
McNair, who has suffered groin problems since returning from a serious long-term knee injury, completed 90 minutes in the midweek defeat to Bolton Wanderers and was handed another start against Middlesbrough at the Stadium of Light.
The Northern Ireland midfielder was involved in a heavy collision midway through the first half and was clearly struggling.
Sunderland will assess his groin injury and expect to know the severity within the next two days, Coleman admits he is a big loss when he isn't playing.
"It was his groin," said Coleman.
"We look a good team with him in the midfield, the physicality of the team changes when he is playing.
"In the second half at one point I was looking at us and them, we are quite small, in the Championship that is not a position to be in.
"We have Paddy McNair in the centre of midfield, that big presence and he has legs, gets round the park. We look a different outfit with him in the side.
"Groin, we won't know for 24-48 hours the severity. He is a loss for us."
The 3-3 draw with Boro leaves Sunderland bottom of the Championship, four points from safety.