Morais was handed his first league start in nearly three months against Scunthorpe United on Monday but the Portuguese attacker was forced to limp out of the Boxing Day encounter in the second period.
Following the goalless draw with league leaders Scunthorpe, McCall's men face Bury on New Year's Eve at the Northern Commercials Stadium before travelling to Northampton Town just two days later.
McCall said: "Fil got a little knock on Saturday.
"It wasn't quite clear at first whether it was cramp or a tweak of something.
"It is a bit too early to say at the moment for him but we are hopeful for the weekend.
"He is being monitored."
McCall says injured striker Billy Clarke is making progress from his calf problem but the Irishman won't be fit to face either Bury or Northampton over the extended weekend.
The City chief added: "He is back in the gym now doing CV (cardiovascular) work.
"He is able to see some light at the end of the tunnel now because he can get a sweat on and really work on his fitness levels.
"There is no getting away from it unfortunately though, calf injuries can be slow to heal and something that can't be rushed.
"There is no chance of him for these next two games (vs Bury, Northampton) but we are hopeful of getting him back out on the training ground sooner rather than later."