Fraser, a strong candidate to tour Argentina with England this summer, went off after just 15 minutes of Saracens' 28-23 Aviva Premiership defeat against Gloucester at Kingsholm.
He suffered a shoulder injury, and Saracens rugby director said: "Will is going to go for an MRI scan, and we will know more after that. It would be a big loss. He has had a great season, but we have got some decent back-rowers to cover."
Saracens' six-game unbeaten Premiership run came to an end after wing Jonny May scored two first-half tries and Gloucester's England number eight Ben Morgan claimed a superb solo touchdown that confirmed Heineken Cup qualification next term.
"It wasn't maybe the performance that we wanted," McCall added, after centre Joel Tomkins scored two tries and his midfield partner Owen Farrell kicked 13 points.
"Gloucester put us under some pressure from time to time. We gave away a couple of reasonably soft scores - some of our mistakes gave Gloucester their opportunities.
"We were 25-16 down, but arguably we could have won that game at the end. We fought our way back into it. The nature of our fightback and the spirit and togetherness is what we are about.
"Toulon is a huge challenge for us, and that is the only game we will talk about for now. There will definitely be things we can take from today's game. Sometimes you need some gentle reminders of what you are about and what makes you successful."
Gloucester boss Nigel Davies was delighted with his team's display as they brought the curtain down on their home games this season.
"We showed elements throughout our game today that we have seen bits of this season," he said. "If we become more consistent in certain basics of our game, then this team is very capable. The players were fantastic, the crowd was superb and everything came together."
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