Scott Sizemore’s season is over before it even began.

The A’s announced Monday the third baseman will miss all of 2012 after he suffered a torn ACL in his left knee.

Sizemore, who batted .249 with 11 home runs and 52 RBIs in 93 games last year with Oakland, sustained the injury in the team’s first full-squad workout Saturday.

He underwent an MRI test Monday and results were examined by Dr. Douglas Freedberg of the Arizona Sports Medicine Center of Scottsdale. Sizemore will be re-evaluated in two weeks to determine when a surgery date can be set.

The 27-year-old Sizemore was acquired by the A’s in May in a trade with the Detroit Tigers.

The injury is a big hit to the already offensively challenged A’s, who hit just .244 as a team last year, 23rd among the 30 clubs.

Sizemore was penciled in as the Opening Day starter for Oakland’s season opening two-game series in Japan against the Seattle Mariners starting March 28.

Eric Sogard, Adam Rosales, Josh Donaldson and Wes Timmons are the top candidates to replace Sizemore.