4:00pm: The Red Sox have officially announced the move. To clear space on their active roster, they’ve optioned pitcher Rubby De La Rosa to Triple-A Pawtucket.

9:13am: The Red Sox are set to promote top prospect Mookie Betts, John Tomase of the Boston Herald tweets. There’s no word yet on a corresponding roster move.

The Red Sox drafted Betts in the fifth round in 2011, and beginning in 2013, he quickly cut his way through the minors, hitting well at each level. The 21-year-old second baseman and outfielder hit .345/.437/.520 in 359 plate appearances in 2014 split between Double-A Portland and Triple-A Pawtucket.

Betts is ranked the No. 51 prospect in the game by MLB.com, No. 61 by ESPN’s Keith Law and No. 75 by Baseball America. BA’s Prospect Handbook 2014 ranked Betts the Red Sox’ seventh-best prospect, praising his speed, athleticism and ability to control the strike zone. With Dustin Pedroia at second base, the Red Sox will likely find playing time for Betts in the outfield, where the team has struggled this season.