Napoli has been dealing with myriad injuries and is still recovering from a dislocated ring finger suffered last month when he slid into second base head first. He also missed two games last week because of the flu.
"Mike was still a few days away," Red Sox manager John Farrell said. "We wanted to give him ample time with the finger, but given the situation and the need for an added arm, we felt like we had to make the move to put Mike on the DL.
"Hopefully this gives him that extended period of time to get past this in its entirety, or at least to try and get further ahead of it."
Napoli is hitting .260 with five home runs with 22 RBIs in 43 games this season for the Red Sox, who have lost 10 straight games and sit in last place in the American League East. CORCORAN: MASH Unit: Building an MLB All-Star team using only injured players