Howell was progressing steadily and threw a simulated game at Tropicana Field prior to Monday night's game, his first time throwing full-speed this year. But he was only able to throw 12 pitches before leaving the mound, clearly unhappy with how the outing went.
"He was upset. I saw that," manager Joe Maddon said. "He was really disappointed."
There has been much speculation that the injury could keep Howell sidelined for the rest of the season. Given his latest setback, the possibility of Howell returning this year appears more remote than before, as season-ending surgery might be an option.
The 28-year-old left-hander has not pitched for Tampa Bay this season, as the weakness in his left shoulder was detected as soon as he reported to Spring Training. Originally, Howell hoped to be back by late May or early June and start a rehab assignment in Port Charlotte, Fla., as early as this weekend.
Tampa Bay's relievers have stepped up their games in Howell's absence, but Maddon said he was still hoping for the lefty to return later this season. The bullpen has compiled a 1.57 ERA since April 14 and has only allowed three earned runs in its last 31 1/3 innings.