Corbisiero suffered a calf injury in the Test win over Australia last weekend, while Roberts was ruled out of the first match with a hamstring problem.
"Alex is struggling a little bit and Jamie is probably in the same boat. Both of them should be fine for the third Test, but at this stage... we have a recovery day tomorrow when everyone's off. Then we'll look at seeing how they are on Thursday morning," Gatland told reporters at a press conference.
"The medics have been doing a fantastic job for us and they might be declared fit, but it's doubtful."
The Lions recorded a 35-0 win over the Melbourne Rebels on Tuesday.