Scotland left-back Wallace was forced to come off midway through the second half of Sunday's Ramsdens Cup final defeat by Raith Rovers while Templeton (groin) and Little (thigh) missed the shock reverse.
McCoist told the club's official website on Tuesday: "Lee has a hip flexor problem at the top of his thigh and he's struggling a little bit at the moment.
"I spoke to him yesterday and again today and we'll give him every chance but I'd have to say there will have to be a dramatic improvement for him to be involved at the weekend."
Pre-match injury doubts Jon Daly, Nicky Law, Ian Black and Nicky Clark came through the 120 minutes at Easter Road unscathed.
McCoist said: "They are heavy-legged and have taken a going over physically but they'll be ready to go.
"We'll look after them this week, we'll do our work and we'll have them prepared for Dundee United. I wouldn't think Andy and David will make it though.
"From speaking to our physio Stevie Walker, there would have to be an unbelievable improvement in them.
"We've been a little bit unlucky with injuries at a really important time of the season but we have to prepare the team as best we can."
Under-fire Rangers can at least take comfort from the fact the semi-final takes place at Ibrox, following the Scottish Football Association's controversial decision to name the venue months in advance.
But Daly claims home advantage will count for nothing when his former team visit Govan with about 11,000 supporters.
"It's a one-off game so I don't think the venue really matters," the Irishman said. "It's a grass pitch and there are going to be 22 players so that's all that's important.
"Of course, it'll be good to have our supporters there and hopefully they'll get right behind us but United will have their fans at the match too.
"When you're on the park, you don't really concentrate on what's happening off it too much. There's a big prize at stake and that's what everyone will focus on.
"I know a lot of people have said there's an advantage for Rangers but I don't see that. It's a game both sides have a chance in."
Bookmakers have made United favourites to win against the League One champions but Daly, who helped them knock McCoist's team out in the last two competitions, also feels that fact is irrelevant.
"I don't think the lads will see themselves as underdogs and I don't think United will look at themselves as the favourites," he said.
"It's just going to be one of those games where you have two good sides having a go at each other and trying to get to a cup final.
"We believe we've a really good pool of players that can do well and I'm sure the manager believes that too.
"It's hard to judge where we are because we're not playing Premiership teams every week and this will be a different type of game to what we've been used to this year.
"But hopefully we can rise to the challenge and get through to the final, which would be fantastic for us all."