The Wales international originally sustained the injury in the Scottish Cup semi-final win over Aberdeen last Sunday and was supposed to miss the Old Firm derby at Ibrox but was passed fit prior to the match.
However, he aggravated the problem just after half-time and had to be replaced by Kris Commons in the 55th minute and is now likely to be sidelined for the season run-in.
"All I've been doing is icing the leg since Sunday,” he told SunSport.
"It's been agony since I came off. I've never known anything like it. I can barely walk.
"I won't know anything until the results come back. I'm a quick healer so you never know. It's not looking good though judging by how it feels.
"It felt absolutely fine on Saturday, so obviously I wanted to play.
"I'll be gutted if I miss the run-in and the Cup Final so I'm hoping for a quick recovery."