The Scottish international re-joined his former club on Monday and made a late appearance from the bench on Tuesday but could not prevent the second-placed Steelmen from slipping to a 1-0 loss at home to Dundee United.
And McCall says the Motherwell support need to be patient with McFadden, who has only managed three substitute appearances for Sunderland this season.
McCall said: "We have to manage the expectations of everyone, including the fans, the board and James himself - he doesn't have a magic wand.
"Over the rest of the season, hopefully, he will get fitter and stronger and be a real asset to us.
"It would have been nice to bring him on in different circumstances. It was a disappointing night, mainly to lose the goal that we did, because we had warned the boys of the danger that Dundee United pose in breaking from corners.
"In the first half I thought we played reasonably well without creating many chances and, as we pushed in the second half, they hit us a few times on the break.
"At half-time I would have taken a point. It would have been a decent point as we weren't at our best - it was a chance missed to put a bit of a gap between us and the other sides."