Hoping to get a better look at Howell, the Rays have penciled in the left-hander to start against the Reds on Friday night in a move that will push staff ace Scott Kazmir into a Minor League start Friday afternoon against Triple-A Scranton/Wilkes-Barre (Yankees) at the Naimoli Complex. Jackson will start Saturday against Boston when the Rays host the Red Sox in a 1:05 p.m. ET contest at Progress Energy Park.
Right-handers Tim Corcoran and Jae Kuk Ryu were the other two starters competing for the No. 5 spot.
The competition is "more reduced down to those two guys primarily," manager Joe Maddon said. "I'm not saying the other guys are not involved. Who knows what's going to happen. I would think everything being equal and if it just continues to proceed the same way, it will be between those two people."
If indeed Corcoran and Ryu are out of the No. 5 starter competition, each will contend for spots in the Rays' bullpen.
A hint as to who might win a close competition between Jackson and Howell might be a transactional matter. Jackson is out of options, which means he would have to clear waivers to return to the Rays' farm system. Howell still has options, meaning the Rays can option him to Triple-A Durham without fear of losing him to another club.
If Howell wins the spot, Jackson would likely be looked at in the bullpen, but if Jackson wins, it's likely Howell would start at Durham rather than go to the 'pen.
Healing hamstrings: Dioner Navarro and Rocco Baldelli have been fighting hamstring problems.
Navarro, who has been battling a strained left hamstring, returned to the lineup to catch three innings Thursday night after having a four at-bat stint in a Minor League game. He did not catch Wednesday.
Navarro said Thursday night's start went as he expected.
"Went pain free, everything was good overall," Navarro said.
Still the Rays want to be cautious.
"Every game we're going to find out what we can do the next time out [with Navarro]," Maddon said. "Assuming that we can go five [innings] the next time, if everything goes as planned."
Meanwhile, Baldelli's return remains up in the air as there has been little change in the condition of the center fielder's tight right hamstring.
"Literally there's nothing new going on," Baldelli said. "I'm just going to keep monitoring it every day when I show up. Get it loose, go out there and see what I can do."
Baldelli said he and Maddon talked about several scenarios for when the center fielder might come back, but "nothing concrete."
After Baldelli started the 2006 season on the disabled list with a left hamstring problem, Maddon is adamant about not wanting to experience a repeat of that scenario in 2007.
"I really want to be overly cautious right now," Maddon said. "I want him to be that way also. I don't want to miss him this year. I saw what he can do [last year]. Gosh, he's just such a high-end player. I want to make sure we can get through this moment and move on and hopefully have no more setbacks through the course of the year."
Baldelli did not play until June 7 last season, but he finished strong, hitting .302 with 16 home runs and 57 RBIs in 92 games.
Roster moves: The Rays reassigned first baseman Hee Seop Choi and catcher Yamid Haad to Minor League camp. Choi signed a Minor League deal with an invite to Spring Training, but Maddon said he simply didn't get the chance to prove himself.
"I really loved the man, we just didn't have enough at-bats for him," Maddon said.
Up next: The Rays will host the Reds on Friday night in a 7:05 ET contest at Progress Energy Park. Howell will start for Tampa Bay and oppose Cincinnati right-hander Aaron Harang.
Bill Chastain is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.