The 25-year-old is yet to feature this term having picked up the injury during pre-season, leaving him doubtful for England's upcoming Autumn Internationals.
McCall refused to put a date on Farrell's recovery, but believes that the fly-half is nearing a return to action.
"Owen trained on Thursday and trained as part of the opposition. That's a good, encouraging sign. The scan he had last week came back positively, so that's good. It's really a case of when can Owen do all the things he needs to play. Once he can, he'll play," he told reporters.
"He hasn't placed kicked, but he's kicking out of hands. I'm not sure if he'll be fit for the Toulon match, but his ability to go straight back in would be a consideration.
"I'm nervous about saying too much about it because he's just done a 45 minute training session and looked pretty good, but it's how he feels over the next 24 hours that's the important thing."
England's Autumn International series begins against South Africa next month.