The injury occurred during Friday's loss to the Tampa Bay Rays. Since Napoli did not play at all on Saturday that means the move is retroactive to Saturday.
Napoli is in the first year of his two-year contract worth $32 million he signed in the offseason.
Napoli this season is only hitting .260, after starting the season off above .300, with five home runs and 22 runs batted in. Napoli's .390 OBP was tops on the team.
Last season saw Napoli hit .259 with 22 home runs while driving in 92 runs. Napoli also only made six errors at first last season.
Starter/reliever, Brandon Workman, has been called up to take Napoli's spot on the roster.
Napoli joins Will Middlebrooks, Felix Doubront and Shane Victorino as current Red Sox starters on the DL.
With the Red Sox offense scuffling, an injury to their #4 hitter is the last thing they needed.
Mike Carp and Daniel Nava are expected to see time at first base for as long as Napoli is out.