City travel to Vale Park at the weekend in Sky Bet League but Marshall is currently a doubt for the game after picking up a hamstring problem against Bolton Wanderers last time out.
Josh Cullen, who is looking for his 50th appearance in City colours, is expected to be fit, however, despite receiving a knock in the same Bolton fixture.
Speaking ahead of the Vale match, McCall said: "Josh has trained today (Thursday) so he should be fine.
"Josh's injury was only a knock, Marshy's is a bit different. He has a tight hamstring - muscle injuries and injuries of that nature can be a bit harder to shake off.
"He has been improving every day and he is confident of being involved but, equally, we have to learn our lessons from earlier in the season.
"Billy Clarke and Tony McMahon both came back a bit too soon and then put themselves out for longer.
"We still have another couple of days to go before the match. If he keeps improving the way he has been doing, we think he will be fine.
"If he is still a doubt going into the game though, we won't be taking any chances."
Alex Jones, who spent the first half of the season on loan at Vale, made a welcome return to training this Thursday after a period on the sidelines with a hip problem.
McCall confirmed once again, however, that the striker won't be in contention for a place against one of his former clubs this weekend.
McCall added: "He joined in with training with us today (Thursday) and had no problems.
"He looked bright, and it was certainly great to have him out there again, but Saturday will definitely come too soon for him.
"He has been out for over three weeks now so he needs to build up his fitness levels again."