"Hopefully that's all, a torn meniscus," Montero said.
Montero will undergo more extensive tests on Monday to determine if there is damage to any of the ligaments in the knee. Until the results of those tests are known, D-backs manager A.J. Hinch declined to estimate a timetable for Montero's return.
"We'll continue to push forward and get him healthy as soon as we can," Hinch said. "Being behind the plate [with] anything [involving] a knee is a stress. We'll see when we get the other tests back. That would be the best-case scenario for him, just a torn meniscus."
To replace Montero on the roster, the D-backs recalled catcher John Hester from Triple-A Reno. Hester arrived less than two hours prior to Sunday's game against the Pirates.
The injury occurred in the second inning of Saturday's 6-3 loss, when Montero hit a tapper back to Pirates pitcher Zach Duke. Montero said that contrary to what was assumed, he did not injure the knee slowing down to avoid Duke's tag in the baseline.
In fact, it was the opposite.
"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."
The loss of Montero will certainly hurt Arizona offensively. Montero, who took over catching duties for Chris Snyder last season when Snyder injured his back, hit .294 with 16 homers in 425 at-bats.
Now, it will be Snyder taking over for Montero.
"You never want to see something like this happen," Snyder said. "I've been there -- it's no fun."
The D-backs nearly traded Snyder to the Blue Jays early in the offseason. In retrospect, it may turn out to be in Arizona's best interests that Toronto nixed the deal at the last moment.
"It's good that we have Snyder and the depth that we've talked about for a long time," Hinch said. "So in that regard, we're very fortunate to have another starting catcher. The stability behind the plate is still intact. Losing [Montero's] bat -- he's a big part of our offense. That's a tough blow. I think Snydes has prepared. He understood the situation. That makes me feel more comfortable, knowing how prepared he's been."
When asked during Spring Training about having a reduced role this year as Montero's backup, Snyder stressed the importance of being ready because, as he learned last year, anything can happen.
"I'm just going to do what I do," Snyder said. "I'm just going to do it a little more frequently."
Montero had an MRI taken Saturday night, but didn't get the results until Sunday morning, which made for some anxious moments.
"I went to bed last night and I couldn't even sleep," he said. "Not because of the pain, just thinking about hopefully it would be fine."
Steve Gilbert is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.