The 18-year-old has not featured for the League One champions since the end of January and it was reported before Saturday's 2-1 win at Arbroath that the viral issue was also affecting his heart muscles.
McCoist said: "Lewis has had an uncommon reaction to a common virus, which is affecting his internal muscles. Some of the muscles around his heart are being affected as well.
"It's nothing more sinister than that. He's getting the best care and attention. You don't mess about with these things until he is 100 per cent right.
"He's seeing the top people but we are quite happy with the way things are going with the way he is improving.
"But until he gets the go ahead for full-time training we will listen to the people who know what they are talking about."
Fraser Aird's late winner handed the Ibrox side their victory at Gayfield but
it was far from an impressive display.
Jon Daly's 25th goal of the season had Rangers ahead after 19 minutes but
Sebastien Faure's slip let in Paul McManus for the equaliser only for Aird to win it with three minutes left.
Daly later hobbled off and McCoist added: "He's felt his hamstring go a bit tight and the doctors are looking at him. We won't be able to assess it properly for another 24 to 48 hours.
"It would be a major blow if he was to be ruled out but I can't really say if that would happen as I'd be guessing. We got him off though before he could do any serious damage."