General manager Jon Daniels says Harrison started feeling pain in his leg. An MRI on Wednesday revealed another herniation in the same disk, and he had additional surgery to resolve that.
Daniels said Harrison came out of the surgery feeling great.
The Rangers don't expect Harrison back for about three months.
Harrison, an 18-game winner and All-Star last year, started the opener this season, the first of his new $55 million, five-year contract. He was 0-2 with an 8.44 ERA in his first two starts while trying to pitch through lower back soreness.