Middlebrooks hit his fifth homer of the season in the third inning off Major League veteran Zach Duke, a three-run shot to center that put the PawSox ahead by a run. He began the game with an RBI single in the first, knocking home Jose Iglesias with a liner to the right side.
The third baseman, ranked as the No. 55 top prospect in baseball by MLB.com, is batting .368 with 16 RBIs and a .390 on-base percentage in 14 Triple-A games.
He has four homers and 13 RBIs in his last six games for the Red Sox and has hit safely in nine of his last 10 starts.
Alex Hassan doubled three times and knocked in a pair of runs while Iglesias, Lars Anderson, Matt Spring and Jonathan Hee all added RBIs in the win.
Harper, meanwhile, continues to struggle as he adjusts to Triple-A -- he went 1-for-2 with three walks and a strikeout and has just one RBI and no home runs in 14 games to begin his Syracuse career. Baseball's top offensive prospect is batting .232, primarily due to his struggles against left-handers: he's hitting .170 (3-for-19) with three strikeouts against southpaws this season.
Chorye Spoone picked up the win in relief for Pawtucket, throwing one scoreless frame. Duke was charged with six runs -- five earned -- on eight hits over four frames, but it was Austin Bibens-Dirkx who suffered the loss when he allowed two runs in the fifth without recording an out.