The injury sustained by Eric Bailly in Manchester United's 2-1 victory over Crystal Palace on Wednesday is a new problem, not a recurrence of his previous knee issue, says Jose Mourinho.
Bailly sustained ligament damage in his right knee in the 4-0 defeat to Chelsea last October and, just three matches after making his return, was withdrawn after 52 minutes at Selhurst Park.
However, Mourinho assured the Ivory Coast international has suffered a new problem, which he tried to play through after receiving treatment in the first half, though he is unsure how long the centre-back will be out for.
Asked how serious Bailly's issue was, the United boss told BT Sport: "We don't know.
"It is not the same knee, it is the other one. It is the left knee. So he is not coming back to a previous injury.
"Of course he has a problem with another dimension but he showed amazing character to try and try, and try until his limit."
United are 13 points behind leaders Chelsea but just six adrift of the Champions League places, and Mourinho is confident they can battle with the teams at the top.
He added: "We are doing enough for that. If we had the points we lost at home in the draws in matches that we did more than enough to win then we would be close to the top of the league, but we lost these points and at this moment we are just closing the gap to the top four positions.
"There are three more matches before the end of the year and let's see how it looks on 31 December."