ANAHEIM -- Unsure of exactly how long they'll be without Jered Weaver, the Angels placed their ace pitcher on the 15-day disabled list Tuesday with a lower back injury that forced him to leave his Monday start against the Yankees in the first inning.

Young righty Garrett Richards, called up from Triple-A Salt Lake, will temporarily take Weaver's spot in the rotation. How long that will be for, though, is still unknown. Weaver hopes to miss just one start, but it'll probably be more like two or three. And, at the moment, it doesn't seem like his absence will extend past one month.

"There's really no timetable," Weaver said. "It's just a matter of how my body responds to everything and how I bounce back from the treatments. That'll tell us when we can get into the stabilizing stuff, the core stuff, to kind of stabilize that part of the back."

A Tuesday MRI revealed Weaver has a strain and some inflammation on the right side of his lower back, near his spine, and is also dealing with spasms. The 29-year-old right-hander reported to feeling better on Tuesday and called the prognosis "the best news I could've gotten."

But it's still not good news, per se.

"It's always disappointing knowing you're not going to be out there pitching the next two or three times," Weaver said. "I'm a competitor, I want to get out there, but I'm going to work my [butt] off in between here to do whatever it takes to get me back on that mound."

The Thursday off-day was originally going to be used to give each starter an extra day off, but now manager Mike Scioscia may skip the fifth spot for the weekend series against the Rangers. That would mean Jerome Williams pitches Friday's game (vs. Colby Lewis), C.J. Wilson starts Saturday (in a rematch with Yu Darvish) and Tuesday starter Dan Haren pitches the series finale (against Matt Harrison).

In the meantime, Richards could be available for the Angels out of their bullpen -- a bullpen that accounted for nine innings in Monday's 9-8 win.

Once the inflammation subsides, Weaver will get back into a throwing program. Scioscia didn't want to go into specifics on a timeline but said Weaver "should be throwing in fairly short order if this thing calms down, according to what the guidelines are."

"I think everyone believes this is going to be very manageable," Scioscia said. "Hopefully it won't be too long, but there's no doubt it's going to take a little bit of time with Jered, and we definitely have to give him enough time to see when he's ready to go back out there and get after it."

Weaver, who's 6-1 with a 2.61 ERA in a year that has seen him throw a no-hitter, felt excruciating pain in his back while firing his 12th pitch to Yankees cleanup hitter Robinson Cano on Monday, then hobbled out of the game after struggling to throw a simple warm-up pitch.

Later, Weaver said it "felt like somebody came up behind me and stabbed me."

Weaver had been on the DL only once in his seven-year career, for a biceps injury that forced him out two weeks at the start of 2007, and has never dealt with any back pain besides the usual stiffness.

"It's just a fluke thing, man," Weaver said. "Just landed wrong, or came off wrong, and just tweaked it a little bit."

Without him, an Angels rotation that collectively sports the lowest ERA in the American League will have to step up. That could also include Richards, the 24-year-old right-hander who came into the season as the Angels' No. 2-ranked prospect, then impressed in Spring Training before a rough May gave him a 4.31 ERA in his first 10 Triple-A starts.

"I don't want him to get too comfortable up here," Weaver said with a wry smile, "but he's going to help out."