The 23-year-old was having a terrific rookie season, hitting .288 with 15 home runs and 54 RBI in 75 games. He was fourth on the team in both home runs and RBI and his .835 OPS was good for third best.
"He had a terrific run with us," Red Sox manager Bobby Valentine said, according to MLB.com. "He's a terrific player. I couldn't be happier or more proud of everything he did developing into the player that he is right now and contributing to our team. It's a shame."
To replace Middlebrooks, the Red Sox called up Danny Valencia from Triple-A. Valencia hit .198 with 17 RBI in 34 games for the Minnesota Twins this season.