"The lat is just not responding the way that he feels or we feel it needs to," Cash said. "We'll just take another couple of days and revisit it again."
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When pressed further for whether Boxberger would be ready for Opening Day, Cash expressed his doubt.
"I just don't see how it's going to happen," he said. "I'm not saying it won't. But it's very tough. Where are we at. ... Realistically, he's a key piece in our bullpen. We're going to do right by him. We want him to get his innings and reps. And it's going to be difficult to do that."
Boxberger made only 27 appearances in an injury-plagued 2016 season, just one year removed from being named an American League All-Star and leading the AL with 41 saves. He endured two trips to the DL in 2016, costing him a total of 99 games.
Without Boxberger, the closing duties will fall to Alex Colome, who closed for the team last season and finished with 37 saves.
Colome was scheduled to return to camp on Monday after pitching for the Dominican Republic in the 2017 World Baseball Classic.
Bill Chastain has covered the Rays for MLB.com since 2005. This story was not subject to the approval of Major League Baseball or its clubs.