Lee was scheduled to rejoin the AL West leaders on Friday.
Lee is 0-3 with an 8.28 ERA in his last five starts. He gave up six runs and 10 hits in 4 2-3 innings Tuesday night at Kansas City.
"After the game last night Cliff Lee told us he was experiencing some discomfort in his back," Rangers assistant general manager Thad Levine said. "We flew him back to Dallas to see (Rangers team physician) Dr. (Keith) Meister. He put him through a battery of tests today and also gave him a trigger point injection in his back."
Levine said some component of the shot is cortisone, "but I wouldn't describe it as a cortisone shot."
"We'll assess where he's at then," Levine said. "We'll know definitely how it feels on Friday. I think he'd been experiencing it before yesterday but he didn't tell us about it until after the game (Tuesday). He did intimate to me last night that he'd been feeling this longer, for a longer period than he told us about."
Lee, the 2008 AL Cy Young winner, has not won since Aug. 6 at Oakland. He is 2-5 with a 4.69 ERA in 11 starts with the Rangers since they acquired him in six-player trade July 9 with the Seattle Mariners.
Levine said Lee has an oblique injury in April with the Mariners, "but we don't think this is related."
Lee's next scheduled start is Monday in Minnesota, but whether he makes that is yet to be determined.
"We'll see how it goes the rest of the way," Rangers manager Ron Washington said. "He didn't have a good August. If he drops three or four wins for us in September we'll forget about that."