PHOENIX -- Ronald Bolaños pitched one inning during his start for Double-A Amarillo on Saturday, then came out of the game and was told he wasn’t going to pitch there anymore.

That’s because, for the first time, the Padres’ No. 15 prospect, per MLB Pipeline, was heading to the big leagues.

Before their series opener Monday vs. Arizona, the Padres selected the contract of the right-handed Bolanos and transferred left-hander José Castillo to the 60-day injured list to clear room on the 40-man roster. Bolanos, 23, signed with the Padres during the 2016-17 international signing period out of Cuba.

Bolanos will make his Major League debut on Tuesday, when he starts against the D-backs.

“We’re excited to have him here; he’s another one of the many young arms we have in the organization and has some really good stuff,” Padres manager Andy Green said. “Very young, limited time in Double-A. We’ve done that quite a bit where these guys have gotten the opportunity here pretty quickly. We’re excited about him, and we think it’s going to be a good learning month for him.”

When Bolanos found out he was heading to the Majors, it caught him off guard.

“If I’m being honest, I was surprised,” Bolanos said through an interpreter. “I wasn’t really expecting it, and I didn’t really believe it.”

Bolanos went 5-2 with a 2.85 ERA in 10 starts for Class A Advanced Lake Elsinore to open the year before a promotion to Double-A in early June. Then he went 8-5 with a 4.23 ERA in 15 games (13 starts) for Amarillo.

Bolanos has shown increased velocity in the Minors this year, as his fastball was up from sitting at 93 mph to 94-95, sometimes touching 96-97. His secondary stuff is not as strong, but he has improved his slider and his curveball has the potential to be an effective pitch.

He had pitched 12 scoreless innings over his previous two starts before the abbreviated outing Saturday. He attributes his recent success -- and his increased velocity -- to the preparation he has put in this season.

“I think routine had a lot to do with this year,” Bolanos said. “I got into a really good one, and it was consistent. It’s been consistent since Day 1.”

Bolanos became the fourth player to join the Padres since rosters expanded Sunday, along with outfielder Travis Jankowski and right-handers David Bednar and Javy Guerra.