"We'll do a scan [on Sunday] but we don't know until we've done the scan".
The Red Devils bought him for £75million from Everton nearly two years ago, though the Belgian hasn't shown the form that he did at Goodison Park.
Three days after scoring twice in a 3-1 victory over Crystal Palace, Lukaku again proved the difference for United with another double.
The Saints shocked Old Trafford when they took the lead when young French full-back Yan Valery scored a scorching goal in the 26th minute.
United were dealt a blow soon after the restart when Alexis Sanchez sustained what could prove to be a significant knee injury, but the home side levelled the game moments afterwards when Pereira curled a pinpoint strike into the top corner from 25 yards out.
Ole's players are now losing 2-0 and have to travel to Paris to attempt to turn the match around.
EPL: Sadio Mane speaks on his new role at Liverpool
There will be more quality and a quicker tempo, but I'll just be helping the gaffer and John wherever I can. Salah starts as favourite, but at around [6.0], Mane would represent better value to open the scoring .
"The dressing room was bouncing, they were happy, looking forward to Wednesday", said the Norwegian.
"We created a lot of chances in the opening 10 minutes but won the game".
"You could see after the 2-1 we didn't stop believing in ourselves and we didn't stop fighting for everything and working as a very good committed team".
"I'm just not sure you are ever going to win the league with a goalscorer like him", said former United midfielder Paul Scholes in November, towards the end of Lukaku's 12-game goal drought in all competitions.
"It's going to be an exciting race and every single game is going to be very important". This has happened before, so many great comebacks.
They travel to Paris Saint-Germain for the second-leg of their Champions League last-16 clash, trailing 2-0 from the first-leg.
"We came back and then one fantastic player decided this game with his unbelievable power and that's the end".
"First five minutes we had a bit of luck that we didn't go back but afterwards we showed that we can also play with the ball and come out of the pressure and press them early", the Southampton boss said.