Anderson Espinoza, the Padres’ No. 1 prospect entering the year, has a torn ulnar collateral ligament and will undergo Tommy John surgery next week. The procedure likely will sideline the 19-year-old right-hander through the end of next season.

Espinoza never made his 2017 debut. Forearm tightness delayed his opening attempt to pitch for high Single-A Lake Elsinore. In late April, the Padres shut him down again. An MRI taken around that time did not reveal ligament damage, and Espinoza resumed throwing in June. He progressed to bullpens this month.

Thursday, in his fourth session off the mound, Espinoza felt a pulling sensation and some pain in his elbow, leading to the latest diagnosis. Dr. Keith Meister will perform Espinoza’s surgery in Dallas.