England loosehead prop Corbisiero and Wales centre Roberts are struggling with respective calf and hamstring injuries that look set to take another week to heal.
"At this stage it's a little bit doubtful that they'll be involved," Gatland said.
Gatland revealed that Corbisiero and Roberts will be assessed on Thursday, when the team for the second Test at the Etihad Stadium will be announced.
"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...," he added.
"We have a recovery day tomorrow when everyone's off. Then we'll look at seeing how they are on Thursday morning.
"The medics have been doing a fantastic job for us and they might be declared fit, but it's doubtful."