"It's already getting better. The starters' pitch counts aren't as high as normal, and we can't string these [relievers] out waiting for it to get better."
Fernando Rodney, Scot Shields and Kevin Jepsen are available to share ninth-inning duties until Fuentes returns. Rodney was 37-for-38 in save opportunities for the Tigers last season and pitched a clean ninth inning for the save in Wednesday's 5-3 win over the Yankees.
"Rodney had a terrific year," manager Mike Scioscia said. "This isn't a huge adjustment of roles for guys. We have depth there. We just need to get guys into their games. It shouldn't slow the chemistry that needs to develop in the bullpen."
Rodney, who frequently has reached triple digits on radar guns, was signed to a two-year free-agent deal this past offseason.
"That's one of the reasons they brought him in," Fuentes said of Rodney. "Obviously, we have other guys who can do it, too. Signing him was a nice insurance policy to have."
Fuentes, who led the Majors in saves last season with 48 over 55 opportunities, worked a perfect ninth inning to save Jered Weaver's win against the Twins in the April 5 opener. Fuentes was in the weight room the following day when he felt the strain in his back.
The Angels haven't had a save opportunity present itself since the opener, going 2-6 with a walk-off win against Oakland.
"I was lifting weights -- I just picked up a dumbbell, and it was a freak thing," Fuentes said. "I didn't have a lot of weights. I saw a chiropractor, and he said he didn't believe anything was structurally wrong. It's a tissue issue. With the rehab we've done, it seems to be getting better."
Also on Wednesday, the Angels recalled outfielder Reggie Willits from Class A Rancho Cucamonga -- where he was rehabbing from a strained hamstring -- and right-handed reliever Francisco Rodriguez from Triple-A Salt Lake. Reliever Bobby Cassevah, who had an 8.31 ERA in three appearances and yielded two earned runs in Tuesday's 7-5 loss to the Yankees, was returned to Salt Lake.
Another roster move will be required on Thursday, when Scott Kazmir will be activated to start the series finale against the Yankees. Kazmir pitched a rehab game at Rancho Cucamonga on Friday after experiencing left shoulder tightness on March 25 and missing a turn in the rotation.