The forward was stretchered off the pitch during the 0-0 draw. And on Monday, Jose Mourinho indicated that the player will miss a week of action.
Normally, a week wouldn't be much to worry about, but the striker has already missed Monday's match against Everton and should miss the FA Cup clash against Derby on Friday. There's also the possibility of missing next Monday's Premier League fixture against Stoke City.
Speaking ahead of the match versus Everton, Mourinho insisted that the striker's absence was simply a method pf protection.
"I would say a week [out for Lukaku]. It's nothing really important - just a little protection, but no problem," the manager said, via manutd.com.
Zlatan Ibrahimovic, meanwhile, is also sidelined with a knee problem, leaving the United squad void of a target man. But Mourinho says he trusts the players he does have at his disposal.
"Of course. If we don't have a player like Lukaku or a normal replacement like Zlatan, we have to play with the qualities we have," he added. "So we have to forget that we have to play without a target man because we don't have [one].
"We have what we have. We trust the players. The players are giving everything like they have done from the first minute of the season. No Lukaku and no Ibrahimovic means we have no no.9 as you normally call a target man that we usually play [with], so we have to change."