Manchester United fear Scott McTominay suffered knee ligament damage on Boxing Day.
United beat Newcastle with a 4-1 score.
During the McTominay was involved in a committed and fair challenge with Newcastle midfielder Matty Longstaff.
"He's done his ligament in the knee, probably," United boss Ole Gunnar Solskjaer said. "I don't know how bad it is, so let's see. The boy has got the biggest heart of the lot, he played until half-time and you stiffen up after. We might have to find out tomorrow to see how it is.
"When they knocked together but you see the attitude of the kid and everyone, to be fair, at the beginning of the game. Not sure if he's gonna be ready for Saturday [against Burnley], probably not, but we've seen him heal quickly before."
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