The Scotland defender hobbled out of Sunday's Ramsdens Cup final defeat to Raith Rovers at Easter Road and has been diagnosed with a hip flexor problem.
Despite initially trying to play on after picking up the knock in the second half, Wallace left the field with just over 20 minutes to go and was replaced by Sebastien Faure in the Gers' shock final defeat.
And now the League One champions face an anxious wait to discover if he will be fit to feature against Jackie McNamara's side at Ibrox this weekend after the 26-year-old reported to Rangers' Murray Park base for treatment yesterday.
The loss of Wallace would be a massive blow to boss Ally McCoist and give him another significant selection headache ahead of an eagerly-anticipated last-four clash.
But Rangers haven't given up hope he will be fit to play after he was added to an injury list that already includes David Templeton, Andy Little and Lewis Macleod as McCoist's side have been hit by a spate of knocks.
Templeton picked up a groin injury in the recent win over Airdrie at Ibrox while Little suffered a thigh strain the same night, but the Northern Irishman hasn't given up hope of featuring against the Arabs on Saturday.
"I really don't know if I'll make it yet, but it would be great if I could," Little told rangers.co.uk
"It's a cliche but I'm taking things day by day. I had a good week last week and I'm going to try again to run this week.
"We'll see how that goes. I don't know either way at the moment, but hopefully I can play a part."
While Little and Wallace still have a fighting chance of facing United, Templeton has stepped up his recovery after working with Light Blues physio Stevie Walker last week, although he was not fit enough to make the squad for the capital cup clash.
Nicky Law, Ian Black Nicky Clark and Jon Daly all featured in the Rovers defeat despite not training last week as they recovered from knocks, but all four came through the final unscathed and will have a huge part to play against United.