The D-backs' catcher has already been diagnosed with a torn meniscus, but the question is whether he also suffered damage to the ligaments in the knee. The team his hoping that's not the case because that will require a longer rehab period.
If it's just a torn meniscus, the timetable for return would be around six weeks.
"He'll get a second opinion from Dr. Yocum and we'll go from there," D-backs manager A.J. Hinch said. "Until then, we don't know."
The injury occurred in the second inning of Saturday's 6-3 loss, when Montero hit a tapper back to Pirates pitcher Zach Duke.
"I was trying to run him over and I felt something pop," Montero said. "I tried to speed up and I felt something pop, and that's when I slowed down."
Montero has been replaced in the starting lineup by Chris Snyder and John Hester was recalled from Triple-A Reno to serve as the backup.
Snyder had a big first day as a starter when he went 2-for-4 with a double, a homer and five RBIs on Sunday against the Pirates.