Vunipola picked up a knee injury, while Farrell suffered an ankle problem during Saturday's 23-6 defeat to Toulon in the Heineken Cup final.
McCall believes that Farrell has a good chance of being fit for next weekend, but Vunipola is a serious doubt for the match at Twickenham Stadium.
"We have some concerns over Mako and potentially Owen," McCall is quoted as saying by ESPN. "We'll see how [Farrell] is. Mako is the big concern for us."
Farrell kicked all six of Saracens's points against Toulon on Saturday.