Macleod was stretchered off with a knee injury in the first half of the Irn-Bru Third Division clash with Montrose at Ibrox on Saturday.
Light Blues medical staff will discover the full extent of the 18-year-old's knock this week, with it unlikely he will be available for the fifth round clash on Saturday.
Templeton, who claimed his side's only goal in the 1-1 draw with Montrose, admitted the talented youngster would be a loss.
But he is convinced that Rangers are strong enough to successfully negotiate the meeting with the Taysiders without him.
He said: "It will be bitterly disappointing if we lose Lewis. He hasbeen brilliant for us this season.
"But Ian Black is a massive player for this club and I am sure he will step up and fill his boots like he did on Saturday.
"Ian was unlucky not to score in the game against Montrose at the weekend after coming on. He hit the post twice in the second half.
"I think Ian just needs a bit of luck, for one of these attempts to go in, and then he will score a good few.
"The game this weekend is against SPL opposition and will obviously be difficult for us. But Ian has vast experience at that level and will play a key role."
Meanwhile, Templeton predicted the Rangers players will have put the 1-1 draw with Montrose behind them when they come to face United at Tannadice.
McCoist's team dominated the Angus club at Ibrox – but they failed to capitalise on the scoring opportunities they created and paid the price at the end.
Temps said: "It was disappointing dropping points against Montrose, but we have to put that to the back of our minds.
"As soon as you get back out on the training ground you have to focus on the next game you have coming up.
"I am sure the gaffer and the coaches will work us hard to make sure that we are ready for the United game at Tannadice."
Right-back Chris Hegarty had to leave the field in the second half against Montrose after suffering cramp in his thighs.
His fitness will be assessed this week.