Ole Gunnar Solskjaer has confirmed Edinson Cavani sustained an injury during Manchester United's 3-1 win over West Ham.
The Uruguayan came on to rescue United against Southampton last weekend, scoring twice and assisting Bruno Fernandes.
He also impressed against Paris Saint-Germain in midweek, despite the defeat.
But Cavani struggled against an aggressive West Ham side in the opening 45 minutes at the London Stadium and was hooked at half-time.
The striker wasn't the only one not to return for the second half - Donny van de Beek was replaced too. Marcus Rashford and Bruno Fernandes came on for the pair.
Where it appears Van de Beek's withdrawal was tactical, Solskjaer has confirmed Cavani's was due to an injury.
Asked about his half-time changes, Solskjaer replied: "Injuries, so it's unfortunate you get muscle injuries.
"Edi [Cavani] had to come off with an injury - hopefully it's not too bad. Anthony had to come off here as well. Of course Bruno is part of this team and 45 minutes is enough."
Fernandes and Rashford made the required impact in the second half with both players heavily involved in two of the three goals.
It was of stark contrast to a dreadful first 45 and Solskjaer explained: "First half we struggled to keep up with the counter attacks and set plays, we had the ball we played with too slower a tempo so we gave the ball away often. Poor first half.
"Second half, of course the goal changes the mood, first one a great goal by Paul. The Mason goal, it's individual quality."
He added: "You know we play against a team that's drawn to City, beaten Leicester, Wolves, draw to Tottenham. They're a good team. They make it difficult for you. They're very compact. They're aggressive. Their pressing is very good, one of the better ones in the league.
"I'm very, very pleased with the performance. Last week we were 2-0 down at Southampton. This was a step forward - only 1-0 down!"