He served strictly as a catcher his first professional season, but his bat (.330, 4 home runs and 37 RBIs for the Northwest League's Tri-City Atoms) prompted the Dodgers organization to try him more in the outfield in 1967 and 1968.

Following Zoilo Versalles' departure in the 1968 Major League Baseball expansion draft, manager Walter Alston shifted Sizemore over to shortstop at the beginning of Spring training 1969.

Brock set a record with 118 stolen bases that season, and credited Sizemore's patience at the plate batting behind him as a big factor.

He was acquired for his versatility,[13] and in the event that a proposed deal sending shortstop Bill Russell to the Cardinals for outfielder Reggie Smith ever materialized (in which case, second baseman Davey Lopes would move to short, and Sizemore would inherit the second base job).

[16] As the game with the Houston Astros went into its tenth inning on August 2, Alston moved Sizemore from second base to catcher for the first time in his major league career.

[18] The National League East champion Philadelphia Phillies were in dire need of a second baseman following All-Star Dave Cash's departure via free agency.

He turned a league leading 104 double plays, while also grounding into a league-leading 25, tying the Phillies single-season record set by Del Ennis in 1950.

He reached the post-season for the first time in his career, as the Phillies duplicated their 101-61 record from the previous season to face Sizemore's former team, the Dodgers, in the 1977 National League Championship Series.

With the Phillies leading 5-3 in the ninth inning of game three, Sizemore committed an error on a throw from left fielder Greg Luzinski that opened the door for the Dodgers to score three runs and steal a 6-5 victory.

[21] The injury sidelined Sizemore until the end of June, and likely played a part in the career low .219 batting average he put up that year.

He, Barry Foote, Jerry Martin and minor leaguers Derek Botelho and Henry Mack went to the Cubs for Greg Gross, Dave Rader and Manny Trillo.

[22] Toward the end of his only season in Chicago, he was dealt to the Boston Red Sox for a player to be named later when Bosox starting second baseman Jerry Remy and his back-up, Jack Brohamer, were both sidelined with injuries.

Sizemore won a reserve job with the Sox out of Spring training 1980,[25] but with the emergence of rookie Dave Stapleton at second base, saw very little action.