SAN FRANCISCO — A day after rookie catcher Daniel Susac was diagnosed with a likely season-ending left knee patellar fracture, the Giants brought back veteran backstop Andrew Knizner on a major-league deal to address the team’s incredibly thin catching depth.

“It’s a good fit for me to come back, and it’s an easy transition,” said Knizner, who caught in 32 games with the Giants last season. “I know most of the guys here. I love playing in the city. It’s a great stadium. I kind of know everything, so it’s quick and easy to just come in and fall into place.”

To make room for Knizner, the Giants optioned catcher Zach Morgan to Triple-A Sacramento. Morgan, a Stockton native, was called up to the majors for the first time in his career on Monday but didn’t join the team until the third inning as he flew in from Las Vegas. He did not appear in Monday’s game.

Additionally, outfielder Victor Bericoto was reinstated from the 10-day injured list, and utilityman Eddys Leonard, who made his debut last week, was designated for assignment.

“Frustrating story conclusion, but a really exciting beginning,” Giants manager Tony Vitello said of Leonard, who went 2-for-8 at the plate. “Obviously, he’s got to forge ahead and hope to lean on this a little bit. At least now, he can say he was able to do some things. He knows what it looks like. He knows he can fit in. Hopefully, a confidence builder and a stepping stone for him in the future.”

Knizner (pronounced KIZZ-ner) was a backup last season to former Giant Patrick Bailey, hitting .221 with a home run and a .598 OPS while worth -1 defensive runs saved. Following Susac’s injury, Knizner and rookie Drew Cavanaugh will form the Giants’ catcher tandem.

“I think it’s good to have his experience, but mostly to have his personality,” said Vitello. “Familiarity adds to that a little bit. He’s pretty quick to get comfortable in a social setting. I think he’s good to have around — good vibes is the easiest way to talk about it. But also, having experience for what lies ahead.”

Knizner had been playing for Boston’s Triple-A affiliate but exercised his opt-out last week (before Susac’s injury), becoming a free agent. With Susac and Jesús Rodríguez (right elbow neuritis) both injured, the Giants reached out to Knizner on Monday. Knizner arrived in San Francisco on Tuesday morning following a six-hour flight across the country with his wife and 10-month-old baby.

“My opt-out was in the middle of last week,” said Knizner, who has hit .294 with 12 home runs in the minors this year. “I’d been playing good. I played myself into what I thought was a good chance to get a major-league opportunity, and this worked out. Each time those opt-outs come up, it’s always good to kind of test the market, see where you stand, especially when you’ve been playing good.”

On the medical front, Susac will rest and rehab for the next four weeks before being re-evaluated, and will seek a second opinion this week. Susac, whose season is likely over, is currently using crutches while wearing a large black brace on his knee while the swelling goes down. He will seek a second opinion this week.

“I’ve blocked balls off my knee and stuff like that, so I thought it was one of those [that was] going to hurt for the night and will be fine tomorrow, ” said Susac, who has never previously used crutches. “Then, it still hurt the next day. When I came to the field, I was like, ‘I don’t think it’s normal.’ ”

Assuming Susac’s year is done, the rookie made a compelling case to be this team’s starting catcher going into next season.

The Giants acquired Susac at last year’s Winter Meetings in a trade with the Minnesota Twins, who had just selected Susac in the Rule 5 Draft from the Athletics. Susac made the Opening Day roster as Bailey’s backup and ended up in a featured role when San Francisco traded Bailey to the Cleveland Guardians.

“I never had any doubt in myself as a player, so that wasn’t a thing for me where it was, ‘Am I meant to be here?’ ” Susac said of his Rule 5 status. “I’ve always believed in myself that way. In spring training, I just wanted to prove it to other people.”

Susac provided tremendous defensive value, ranking sixth in Fielding Run Value (+7) despite being on the injured list three times. Bailey, for comparison, has a Fielding Run Value of +4 despite playing significantly more innings. At the plate, Susac hit .243 with three home runs and a .664 OPS. All in all, Susac was worth 1.1 WAR, per FanGraphs.

“Defensively, I think I’ll be the first one to tell you I think it was the biggest place I made strides at this year,” Susac said. “That’s a credit to [catching] coach Alex Burg. Best coach I’ve ever had. Just really made me so much better and a person. He’s a great person as well. There’s obviously things you can clean up everywhere and keep getting better.”

Along with Susac, third baseman Matt Chapman underwent successful “bilateral core muscle” surgery Tuesday morning in Philadelphia and will travel back to the Bay Area later this week.