McFadden made a late appearance from the bench after signing for the Steelmen on Monday evening, but was unable to cancel out Johnny Russell's opener after 17 minutes.
McCall has now asked for patience from the Motherwell support in regards to the Scottish international, who up until Tuesday evening had only managed three substitute appearances for Sunderland this season.
"We have to manage the expectations of everyone including the fans, the board and James himself - he doesn't have a magic wand," said the Motherwell boss.
"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 - we had warned the boys of the danger that Dundee United pose in breaking from corners."
Motherwell could have lost by more but for an inspired display by Darren Randolph in goal.
Gary MacKay-Steven turned in an excellent performance and will consider himself unfortunate not to have got on the scoresheet.
"In the first half I thought we played reasonably well without creating many chances and as we pushed in second half they hit us a few times on the break," McCall added.
"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."
- Sports & Recreation