SAN FRANCISCO -- The Giants were without two key pieces of their lineup for Friday night’s Bay Bridge Series opener against the A’s at Oracle Park.

First baseman Brandon Belt landed on the injured list for the second time this season with right knee inflammation, and catcher Buster Posey was scratched from the starting lineup with low back tightness. LaMonte Wade Jr. was recalled from Triple-A Sacramento to replace Belt on the 26-man roster, and backup Curt Casali started behind the plate in place of Posey.

The severity of Posey’s malady is unknown, but manager Gabe Kapler said the Giants are expecting to be without Belt for longer than the minimum 10 days, a significant blow for the National League West-leading club.

Belt, 33, tweaked his right knee while running the bases in the Giants’ 9-3 win over the Angels at Angel Stadium on Wednesday and underwent an MRI exam on Thursday. The Giants have yet to complete their “exploration process,” but Kapler said there are concerns that Belt could need surgery on his ailing knee, which also required a microfracture procedure in 2018.

“I think as we evaluate it, everything could potentially be on the table,” Kapler said. “I think the concerns are there, yeah.”

Belt is batting .253 with an .875 OPS and 11 home runs over 55 games this year, but he’s battled various ailments throughout the season, including a left oblique strain that forced him to miss 11 games. The knee injury is likely to sideline him for longer, though.

“He’s disappointed,” Kapler said. “I think he understands that this is probably not a 10-day injury. He understands that there’s going to be a rehab process here, and I imagine that’s difficult. I saw him coming into the ballpark as I was leaving the ballpark yesterday. He wasn’t moving around great. I asked him how he was doing, and he was honest. I don’t think he’s in a particularly good space around this, but he also understands it’s important that he keeps a mindset that he’s going to help us a lot this season, and that the beginning parts of this rehab are going to be important for him to hit the ground running.”

With Belt unavailable, the Giants will likely have Wade and Darin Ruf form a platoon to help fill their void at first base. Ruf started against left-hander Sean Manaea on Friday, but Wade will likely see plenty of time there against righties.

Wade is primarily an outfielder, but he played first in college and has shown that he can handle the position capably. The 27-year-old is hitting .265 with an .814 OPS and four home runs over 26 games this year, but he was surprisingly demoted to Triple-A Sacramento to clear roster spots for Ruf and Alex Dickerson, who were activated off the injured list on Monday.

“It was a professional conversation,” Wade said. “I understood where [Kapler] was coming from and what was going on here with the team and everything. No hard feelings. You just appreciate the fact that you were able to do something while I was here and just look forward to when you come back. Unfortunately, I’m coming back on a sour note with Belt getting hurt and everything. You never want to come back like that, so you hope he’s doing good and hope he can come back because we really need him. With that being said, you try to step up and help out as best you can while he’s gone.”