Ole Gunnar Solskjaer acknowledged his decision to introduce Marcus Rashford as a substitute against Wolves "backfired" as the Manchester United star hobbled off to leave his involvement in Sunday's clash with Liverpool in doubt.
Rashford came off the bench three minutes before Juan Mata scored the only goal of Wednesday's FA Cup third-round replay win at Old Trafford.
But the England striker lasted little longer as he went down following a challenge with Matt Doherty, attempting to play on with an apparent back problem before he had to be taken off 10 minutes from time.
Solskjaer, who said there were "no updates" on a transfer breakthrough, confirmed Rashford had been dealing with a back problem, and the manager was hopeful his risky change would not cost United ahead of a trip to Anfield.
"Yeah, that backfired," Solskjaer told BT Sport . "But you've got to go for them once in a while.
"It seems like he's got a knock there and he couldn't run. Let's test him the next couple of days and hopefully he'll be okay for Sunday. I don't know.
"He's struggled a little bit with a couple of knocks with his back lately, and he got another one which didn't help."
Goalscorer Mata said of team-mate Rashford: "Hopefully it's not too bad and he can be ready [for Liverpool].
"He's on fire, scoring in every game. It was a shame that he couldn't be on the pitch for more minutes, but hopefully it's nothing."
The midfielder added on the cup win: "[Wolves] are a difficult team. They play in a certain way and they do it very well. They are very well organised and difficult to break down.
"But definitely today is a good result for us. It's been a good couple of days and now we have a big one coming, so hopefully we can keep it up."