Castillo, 25, experienced arm tightness on Tuesday, according to the report, and doctors then found the tear in his ulnar collateral ligament.
The Venezuelan left hander has suffered a string of injuries in the last two years, including MLB reported, a left lat strain, a left flexor strain and an injured middle finger.
He arrived in the Major Leagues in 2018.
Another pitcher Javy Guerra, will miss time due to UCL issues, MLB reported. He’s out up to four weeks with a sprain in his right elbow.
Meanwhile, the Padres defeated the San Francisco Giants 9-3 Friday as spring training continues in Peoria. Local product Joe Musgrove made his first appearance, pitching two scoreless innings.
The Friars face the Los Angeles Dodgers at noon Saturday, with Chris Paddack taking the mound against their National League West rivals’ new star acquisition, Trevor Bauer.
The Padres have a 2-4 record in Cactus League action so far.