Haren felt it while pitching six-plus innings of one-run ball in a win over the Rays on Friday, and now it looks as if he won't make his next scheduled start against the Rangers on Wednesday.
Angels manager Mike Scioscia stressed Monday that the current pain is "nothing connected" with the lower-back stiffness that placed Haren on the disabled list earlier this month.
"It's really nothing like it was before," Haren added, pointing to the fact that his back injury was more of a structural issue than a muscular one, like this.
Asked whether Haren will start Wednesday, though, Scioscia said: "Right now, we're not sure."
Haren threw off flat ground prior to the opener of a four-game series at Rangers Ballpark on Monday night, though he was in the trainer's room while his teammates were on the field for batting practice, and Scioscia said he'll "probably" throw a bullpen session on Tuesday.
If he can't go the following day, it'll be Garrett Richards making a spot start.
"It doesn't bother me when I twist, but getting it extended, I'm asking it to do a little bit more than it's done in a while," Haren said. "The day after I pitched I was really sore, and I've made a lot of progress in the last couple days."
Haren is 8-8 with a 4.59 ERA in 19 starts this season. After a rough start in Cleveland on July 3, the 31-year-old right-hander finally admitted that he'd been suffering from lower-back stiffness since the start of the regular season and he went on the DL for the first time in his career. Three weeks later, he was back, and in two starts since, he's given up three runs in 12 innings.
The Angels don't believe his current back issues are enough to put him on the shelf again.
"He feels much better today, and we think that if he does not start on Wednesday it'll be shortly after that," Scioscia said. "We'll see how that goes."