Padilla, who leads the Rangers with 12 victories and has a 4.98 ERA in 25 starts, couldn't throw bullpen sessions Tuesday or Wednesday.
"He's not going to make it Friday," Rangers manager Ron Washington said. "This is frustrating. We want to win as many games as we possibly can."
Washington said the Rangers are not planning to put Padilla on the disabled list, instead hoping he can make his next scheduled start Sept. 3 against Seattle. Plus, rosters can expand to 40 on Monday.
Padilla was put on the disabled list in July with a stiff neck and missed an August start with the injury.
"We'll get [Dustin] Nippert prepared for Friday," Washington said
Dustin Nippert, 1-3 with a 7.83 ERA, is scheduled to take Padilla's turn. His only start among 15 appearances this season was in Padilla's place Aug. 17. He took the loss, giving up seven runs on eight hits, including two home runs, in four innings in Tampa Bay's 7-4 victory.
Nippert's longest and best outing of the year was July 7 against the Angels, when he allowed one run on four hits in seven innings of a 9-6 Texas loss. He made 10 starts earlier this season with Triple-A Oklahoma.
In another move, the Rangers announced outfielder Jason Ellison, who was designated for assignment Monday, has cleared waivers and accepted an assignment to Oklahoma.
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