The move is retroactive to April 5.
Middlebrooks hit .231 with one homer in Boston's first four games before being scratched from the lineup for Saturday night's 7-6 loss in 11 innings to the Milwaukee Brewers. Jonathan Herrera started in his place and was in the lineup again Sunday for the finale of the three-game series.
Holt played 26 games for Boston last season with 17 starts, all at third base. He batted .203 with 11 RBIs.