BOSTON -- Red Sox slugger J.D. Martinez was out of the starting lineup Sunday for the fourth straight game because of back tightness.

Boston manager Alex Cora said Sunday that Martinez "is feeling better, but he's not ready to play."

Martinez, 31, has not started eight of the past 20 games. He missed four games in Toronto from May 21-23. Cora said the first two games were because of illness and the last two because of his back.

Martinez hit .330 with 43 homers and a major league-leading 130 RBIs last year after signing a $110 million, five-year contract.

He is batting .296 with 12 homers and 33 RBIs this season -- down from .318 with 21 homers and 54 RBIs at this point last year.

Boston also was without first basemen Mitch Moreland (strained right quadriceps) and Steve Pearce (lower back strain) as the Red Sox fell to the Tampa Bay Rays 6-1.