General manager Jon Daniels said that Harrison was diagnosed with significant nerve irritation in his vertebrae. He will have an injection Friday in an attempt to alleviate the symptoms before considering other options, including surgery.
Harrison, who made only two starts last season before two operations on a herniated disk in his lower back, came out in the second inning of his start Tuesday night at Houston. Daniels said the pitcher had a tingling sensation down his left leg, similar to what he experienced before having surgery last year.
Perez has a partial tear of his ulnar collateral ligament in his left elbow. He could have an injection and rehab that could keep him out three months, or possibly have Tommy John surgery and miss a year.
Right-hander Nick Tepesch was recalled from Triple-A Round Rock for his scheduled start Wednesday night against Houston. The Rangers also purchased the contracts of right-handers Scott Baker and Miles Mikolas from Round Rock, and designated right-hander Justin Germano for assignment.
Tepesch was starting in place of Yu Darvish, who threw 126 pitches in his near no-hitter Friday night. The Japanese ace was given extended rest and is set to start Friday at home against Toronto.
When Tepesch took the mound against the Astros, he became the 22nd different player to pitch for the Rangers this season and the 38th player overall. Both are the most in the majors.
Texas has 13 players on the disabled list. Among the other hurt Rangers is left-handed starter Derek Holland, who is rehabbing from offseason left knee surgery after being tripped by his dog on the stairs in his home.
Holland, who has been throwing live batting practice at the Rangers complex in Arizona, is scheduled to be in Arlington this weekend for an evaluation and to throw for Rangers coaches. He is still several weeks away from a possible return.
Perez allowed six runs and 14 runners (nine hits, four walks and a hit batter) in 3 2-3 innings of an 8-3 loss to Boston on Saturday night, and reported discomfort in his elbow the following day. Since the end of his 26-inning scoreless streak over three games, Perez (4-3) has given up 19 runs over 13 1-3 innings in his past three games.
Rangers team physician Dr. Keith Meister diagnosed the partial tear for Perez. Dr. James Andrews also reviewed the MRI and agreed that was the issue.
Harrison was examined Wednesday by Dr. Drew Dossett, who did the two operations last year.
Daniels said there was no evidence of disk herniation, but that the spondylolisthesis Harrison is now experiencing was considered a possible risk after the operations - ''If it didn't work as you would hope, this would be the outcome. Unfortunately, that's come to pass,'' the GM said.
The options for Harrison included rehab and to live with the problem or have surgery that Daniels described as spinal fusion.
Daniels said players have returned to the major leagues after have such fusions, but there are no guarantees.
''It's more involved surgery and changes of coming back are lesser than some of the more traditional baseball surgeries,'' Daniels said. ''If he did opt to have surgery, he would be out for the year with a chance to rehab and come in and compete in spring training.''
Infielder Donnie Murphy (neck strain) and left-hander Joe Saunders (left ankle) are scheduled to begin injury rehabilitation assignments with Round Rock on Thursday night. It will be the first rehab game for Murphy and the third for Saunders, who made only his first start of the season for the Rangers.