PORT CHARLOTTE, Fla. -- J.P. Howell is on the sideline for now, but the Rays' stellar lefty is pleased with the progress he's making on his way back from a tired left shoulder.

"I'm way ahead of schedule right now," Howell said. "I'm very happy right now. It's easy to sleep knowing I'm making progress."

Just over a week ago, the Rays announced that the strength in Howell's left shoulder was not up to par, which caused enough concern to have him visit the team orthopedic doctor, Koco Eaton. Tests were negative for any major damage, but they did reveal quite a bit of inflammation. Thus, the prescription for Howell's return was to calm the irritation and strengthen the tired shoulder.

Howell has been working with a weighted ball and rubber tubing, along with a lot of cardiovascular activity.

"It's more of a tedious workout than it is getting tired," Howell said. "It's more mental. The rehab we're doing is something I needed specifically for [the tired shoulder]. We've done about four sessions, and it's improved a lot."

The early results from those workouts have been encouraging. Howell said the strength in his shoulder has improved to a level "barely below what I was in 2008" and he has increased his range of motion considerably.

Howell will continue his workouts throughout the coming week and will undergo another strength test on April 5.

"And that will be a real test to see if I could start throwing really soon or not," Howell said. "If my strength goes up a lot again, it's definitely start throwing. If it stays the same, we'll do maybe one more week of tubing and weighted ball workout."

If Howell is deemed ready to begin throwing, he would throw from 45 feet for a couple of days, then progress to levels of 60, 90 and 120 feet before throwing from a mound.

"Then I'd probably do three or four bullpen [sessions], and then I'd do a live game, see how I feel then," Howell said. "That would be the real test, throwing in a game."

Skepticism about when Howell might return has followed the announcement that he would start the season on the disabled list. He is a critical piece to Tampa Bay's bullpen, so he is tough to replace. Andrew Friedman, the Rays' executive vice president of baseball operations, has been looking at external and internal options to fill the void, but he is looking for the same relief help most clubs in baseball are seeking.

In all likelihood, the Rays will have to plug the gap from within their system until Howell returns. On the bright side, Howell believes that return will come sooner than later.

"I think the worst-case scenario will be late May," Howell said. "I really think that's my worst case. I feel late May is realistic."

Howell went 7-5 with a 2.84 ERA and 17 saves in 69 appearances in 2009 after posting a 6-1 mark and a 2.22 ERA in 64 games in '08.