The A's infielder, 28, initially felt pain in the knee while chasing a fly ball in Tuesday night's game in Anaheim, which marked just his second start since September 2011, and he underwent an MRI administered by team orthopedist Will Workman in the Bay Area on Wednesday that revealed the tear.
Workman and the rest of the team's medical staff are discussing when Sizemore will undergo the surgery, which will be his third in the last five years. Aside from last year's procedure, the infielder also underwent surgery for a broken left ankle in 2009, shortly after the Tigers had handed him the monumental task of replacing Detroit fan favorite Placido Polanco.
Sizemore was traded from the Tigers to the A's in June 2011 and was immediately moved to third base, a position he took to with ease. He also was responsible for Bob Melvin's first win at the A's helm on June 10, 2011, when he provided the club's ninth-inning go-ahead run in Chicago.
Understandably, the two share a special bond, and Melvin was visibly shaken on Wednesday afternoon when relaying the news to reporters.
"Unbelievable," Melvin said. "It's just awful. I've been with Scotty since I took over here. We actually came up around the same time. He had a great year, played out of position, and he's one of those guys you really pull for, because he's such a good guy and cares so much and wants to win. He's coming off two difficult surgeries in the last several years, and to have a third one is devastating.
"We're thinking about him, but I can't imagine what's going through his mind right now."
With Sizemore out of the mix at second base, where he was platooning with Eric Sogard, the A's will continue to rely on Sogard, as well as switch-hitter Andy Parrino, who was recalled from Triple-A Sacramento on Wednesday to take Sizemore's place on the active roster.
The 27-year-old Parrino hit .368 with one home run and nine RBIs in 26 games with the A's this spring, but he struggled with Sacramento before his promotion, having batted .174 in six games with the River Cats, all at shortstop.
"Down there, things weren't really going our way, and you maybe try to press too much, do a little too much," Parrino said. "I think, me personally, that's what was happening, so I'm trying to get back on track and have a better approach at the plate."
Parrino is very much attractive to the A's because of his versatility. Outside of shortstop and second base, he can also play third and the outfield. In fact, his Major League debut -- in 2011 with the Padres -- came in right field.
"He's a good fit for us," Melvin said. "He had a good spring. He switch-hits and is very versatile. He's one of those guys that can really pick you up from any spot."
Added Parrino: "It's great to have the opportunity. It's a great honor and privilege, and hopefully I can jump right in there and help the team any way possible."
Melvin assured there was discussion regarding the possibility of bringing up Jemile Weeks from Triple-A to replace Sizemore -- "There was, absolutely," he said -- but the second baseman can really only play one position, unlike Parrino.
Perhaps not coincidentally, Weeks made himself available at shortstop on Wednesday and started there for the first time in his professional career.
"I wouldn't be surprised if that was Jemile's idea," Melvin said. "It just opens up some versatility options for him."