Two weeks since their last Sky Bet League One fixture, City return to league action at the weekend against Charlton at the Northern Commercials Stadium, but Clarke will have to wait a bit longer for his next involvement.
Clarke has missed City's last four league and cup matches, with McCall saying that the forward is now targeting a comeback in the 'next couple of weeks'.
McCall said: "We only have one injury at the moment - Billy Clarke.
"Calf injuries can be slow to sort out and they can cause further problems if you come back too soon.
"This is a frustrating period for him, understandably so, but we hope he will be back at some stage in the next couple of weeks."
While Clarke will miss out, Tony McMahon is expected to be contention for a place in City's match-day squad against Charlton after his run-out against Cambridge United in the Checkatrade Trophy on Wednesday.
McCall added: "As much as he has been doing fitness work recently, he needed match minutes so it was pleasing to see him get 65 minutes under his belt.
"He came through unscathed and put in a really good performance.
"He is training with us all the time now and he will be back involved in the squad for Saturday.
"A number of the lads who played 90 minutes on Wednesday and who we may look to play on Saturday - the likes of Nicky Law and Mark Marshall - all came through the game fine and have been in for a cool-down today (Thursday).
"The crowd will be buoyant on Saturday and I'm sure that will lift the lads again."