Harrison had surgery eight days ago to repair the disk and felt discomfort the past few days while he was at his house. He had an MRI on Tuesday morning, which showed another piece of the disk had herniated. Doctors went ahead with more surgery.
Rangers general manager Jon Daniels said the club is still hopeful that Harrison can pitch again this season. The timetable for his return is at least three months.
"That's Matt's expectation," Daniels said. "We have to get him healthy."
Harrison was the Rangers' Opening-Day starter and pitched in two games before he began to feel pain in his lower back. Harrison was placed on the 15-day disabled list on April 11.
Harrison had been feeling well since the surgery, but started to feel the same pain down his left leg that he did before his first surgery. He had been doing an hour of rehabilation work a day since his surgery, mainly walking. He couldn't pick up more than 10 pounds.
Harrison, an 18-game winner last season, signed a $55 million, five-year contract during the offseason. He was affected by the injury this season, going 0-2 with an 8.44 ERA as he tried to pitch through the soreness