LAKELAND, Fla. — The Phillies starting rotation thinned Friday just four days after it was bolstered with the addition of Jake Arrieta.

Jerad Eickhoff will start the season on the disabled list and will be sidelined until the beginning of May because of a strained muscle in his back. Eickhoff felt discomfort toward the end of his start on March 9 and has not appeared in a game since. General manager Matt Klentak said the pitcher has a mild strain of his right lat muscle.

The righthander returned to Philadelphia for more testing and will miss roughly six to eight weeks. The Phillies do not believe it is related to the injures — back stiffness and nerve irritation in his hand — that derailed Eickhoff last season.

"Right now, there doesn't seem to be any connection between last year's injury and this year's injury," Klentak said.

The Phillies do not plan to acquire a pitcher to replace Eickhoff and instead will focus on four of their internal options: Zach Eflin, Ben Lively, Mark Leiter Jr., and Tom Eshelman. Those pitchers were competing for the fifth spot in the rotation until Arrieta arrived this week. That competition now resumes. The Phillies still remain unsure if Arrieta will be ready to start the regular season, but they can navigate the first 11 games with four starters before needing a fifth pitcher on April 11.

"We feel very well-positioned to support this," Klentak said. "This is not a long-term thing. We've been talking for a while that we have a lot of candidates to take the ball at the big-league level. So we'll be fine."

Eflin seems to be the favorite, as he has looked strong this spring. The righthander is finally at full strength after surgeries he had on both knees at the end of the 2016 season. He pitched last season on thin legs because he did not have time to build back muscle lost during his recovery. His pitches look crisp, and he seems confident on the mound.

"It's honestly unbelievable," Eflin said. "I've never felt like this in my life. I can actually utilize my legs out there. It's night and day from last year. Putting more pressure on my legs makes my arm feel so much better than it used to. I'm not sore after outings. It's really a huge step for me. I feel explosive on the mound."

Eickhoff will not resume throwing for another seven to 10 days. He will then have to restart his spring-training program, meaning his return will likely be, at the earliest, in mid-to-late May. He was steady two seasons ago, making each of his 33 starts and logging a full season as the the team's other young starters were limited by injuries or innings limits.

But he struggled last year as he tried to pitch through injuries before being shut down for the final month after making just 24 starts. Eickhoff said he was fully healthy when he arrived to spring training, but something did not seem right when he was roughed up in his last start. The Phillies found out Friday that something wasn't.

"There might be a blessing in disguise here," manager Gabe Kapler said. "We're always thinking about keeping guys healthy and strong and limiting their total innings count. Those are things that are always on our mind, so it's possible the innings are limited on the front end and then in September, October, he's strong and healthy and prepared to go through a full season."

Extra bases

The Phillies added utility player Pedro Florimon to the 40-man roster and designated infielder Eliezer Alvarez for assignment. The move was made because Florimon had the option to become a free agent if he was not added to the roster. The team's competition for the final two bench roles is still open. … J.P. Crawford homered in Lakeland and Nick Pivetta struck out five in five innings as a Phillies split squad tied the Tigers, 6-6. … Vince Velasquez racked up five strikeouts in Clearwater but was troubled by a high pitch count as he lasted just 2 2/3 innings in a 6-4 loss to the Blue Jays. Cesar Hernandez hit his first homer of the spring. … The Phillies will wear their St. Patrick's Day green jerseys and caps Saturday when they host Atlanta.