The Belgium international, a 75-million-pound signing from Everton in July, collapsed after banging his head against Southampton's Wesley Hoedt in an aerial challenge early in the game and immediately received medical attention.
He appeared unable to move as he lay on the pitch and after a few minutes was taken off on a stretcher in the 14th minute wearing an oxygen mask. He was replaced by England striker Marcus Rashford.
Lukaku, 24, has scored 15 goals in all competitions for United this season. Before the injury, he came close to giving his side the lead, heading a cross from Juan Mata a few inches over the crossbar.
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United coach Jose Mourinho could not give any further details on Lukaku's condition but appeared to rule him out of Monday's game against his former club Everton and Friday's FA Cup tie at home to Derby County.
"I don't know, I know that normally bad, bad news arrives immediately and bad news didn't arrive," he told a news conference.
"But to leave the pitch the way he did, and we didn't think twice about making that decision immediately, is because there is a problem not just for today but the next two matches I would say for sure."
(Reporting by Richard Martin,; Editing by Rex Gowar and Pritha Sarkar)
