The 24-year-old, named in England's 33-man squad for a training camp in Oxford next week, damaged his knee during the first half of Sarries' 41-13 victory at Allianz Park on Saturday afternoon.
England boss Eddie Jones will now be sweating over the fitness of the 34-cap ace, who appears to be a major doubt for the Autumn Internationals against Argentina, Australia and Samoa in November.
"It didn't look great and I hope it's not as bad as it looked, but he's gone for a scan," McCall told reporters. "It's the same knee he injured for England against Argentina last November and it's cruel luck as we obviously feel for him terribly."
McCall also revealed that fellow back-row forwards Michael Rhodes and Schalk Burger will be out for three months and four weeks respectively with shoulder injuries.