Rafa Benitez is hopeful that Salomon Rondon is not facing a long spell on the sidelines.
Rondon, forced off against Crystal Palace last weekend with a thigh injury, is out of tomorrow's home game against Leicester City.
United manager Benitez said: "He cannot play. We have to assess him in the next weeks and see how he is going on. The other players are training and we will have to make a decision later."
Rondon joined the club on a season-long loan from West Bromwich Albion in August.
“I was talking with him this morning," said Benitez. "When he came, he came a little bit late . He was injured when he was at West Brom. Then after when he was improving his fitness – it was obvious he needed to improve hit fitness – he went with the national team for 15 days, so we couldn't have the control.
“Coming back, hard training sessions, then he felt something in the warm-up the other day, so we were watching the game and we said 'it's not fine' and we had to make the substitution.
“People watching we saying he could not do this, but he couldn't. He was trying – because he had not been playing, he wanted to impress, but he couldn't and that's it."
Asked for a timescale on Rondon's return, Benitez said: “We need to check. I think a couple of weeks, maybe. The idea is to see how he is going on after every week."