The right-hander went on the disabled list earlier this month for the same problem, which has apparently hampered him all year. He was set to pitch Wednesday against Texas.
Haren said the discomfort is not in the same place as his pain when he was placed on the DL. Haren described it being on his side around his lower rib cage.
"Talking to the doctors, they said it was very common for someone with back issues to feel some pulling down there," Haren said. "If you get an inch or two more extension, you're going to get some pull. 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.
"The day after I pitched I was really sore. I've made a lot of progress just in a couple of days. I don't know what my status is exactly."
Haren -- who holds an 8-8 record with a 4.59 ERA this season -- threw long toss from about 200 feet on Monday and hopes to throw a short bullpen session on Tuesday.
"I'm going day by day," Haren said. "I've put in four or five hours a day in the training room and have made a lot of progress. ... Tomorrow I might try to get out on the mound. It just goes by how I feel."
Meanwhile, outfielder Mark Trumbo missed Monday night's 15-8 win at Texas because of back spasms. He felt pain during batting practice on Sunday.
Information from ESPNDallas.com's David Collier and The Associated Press was used in this report.