Diaz was placed on the injured list on July 23 with a left foot injury, which occurred after he fouled a ball off the foot a day earlier. He has been on the injured list three times this season.
He is hitting .270 with 14 home runs and 38 RBIs this season.
Sulser, a 29-year-old right-hander, appeared in seven games out of the bullpen this season and didn't allow a run.
The Rays, who play their final game of the season on Sunday, will face the Oakland Athletics in the American League wild-card game Wednesday.