The injury also sidelines him for Scotland's upcoming Nations Championship clashes with Argentina, South Africa, and Fiji this summer.
"Alex Craig is our first casualty," said Glasgow head coach Franco Smith. "He won't be able to play again this season. He'll be back only for round two of URC 26-27. He ripped a tendon at the bottom of his foot underneath his big toe. He actually thought his boot had ripped but he changed the boot and the pain didn't go away.
"Obviously it's a blow for us. He's going to get operated on Monday and hopefully we'll get the recovery process going as quickly as possible."
Craig joins fellow Scotland locks Scott Cummings (calf) and Gregor Brown (hamstring) on the injury list. Cummings could return within a couple of weeks, while Brown - initially expected back in Jun - remains unavailable until at least the URC knockout stages.
"Everything in rugby happens for a reason," Smith added. "We're going to get Gregor and Scott back at the time when we need them most. They are progressing well, both of them. Hopefully we can survive a couple of games more without them and they will be able to take the baton over when we need it most."
Kyle Rowe will also miss Saturday's game with Bulls with a hip injury, but Smith was confident he would be available should his side make it into the Champions Cup quarter-finals.