HOUSTON -- The Astros tabbed one of their prized pitching prospects to make his Major League debut and start Monday's game against the A's when they announced Friday that right-hander Lance McCullers Jr. was coming up after dominating in Double-A.

McCullers, 21, was actually promoted to Triple-A Fresno on Thursday, but with the Astros needing a starter on Monday after an injury to Brett Oberholtzer, the team turned to McCullers, who had yet to appear in a game for the Grizzlies.

"It was amazing, it was surreal," said McCullers, who was told of his promotion by Fresno manager Tony DeFrancesco during a team meeting Friday. "I kind of didn't really process it for a couple of seconds. Tony told me, and I was like, 'What?' I didn't know how to handle it or process it. It was so unexpected. It was special, for sure."

McCullers, who wasn't on the 40-man roster, went 3-1 with a 0.62 ERA in six games (four starts) for the Corpus Christi Hooks, allowing 15 hits and two earned runs in 29 innings. He struck out 43 batters and walked 11. Corresponding moves will be made to make room for him.

"A lot of factors went into it, but Lance has been pitching exceptionally well," Astros general manager Jeff Luhnow said. "We had really good feedback from everybody who's seen him in Spring Training through every one of his outings this year, and the idea was we'd promote him to Triple-A and he was going to start Monday at El Paso. When Obie came up with the blister, it made sense to call him up. It gives us a chance to win ballgames right now in Houston, and we're going to take advantage of it."

Currently ranked by MLB.com as the Astros' No. 10 prospect, McCullers will be youngest Astros player to make his Major League debut (21 years, 228 days) since Jose Altuve on July 20, 2011 (21 years, 75 days). He was taken with the No. 41 overall pick in the 2012 Draft out of Jesuit High School in Tampa and was Luhnow's second Draft pick (behind Carlos Correa) and is the second Luhnow Draft pick to make it to the Majors (behind outfielder Preston Tucker).

"We believe he's got the repertoire and stuff to pitch in the big leagues and be successful," Luhnow said. "There are other pitchers we have in Triple-A and Double-A that are close, but when we looked at our options, he gives us the best chance to win on Monday."

Upon hearing the news, McCullers immediately tried to call his dad, Lance McCullers, who pitched seven years in the Major Leagues with the Padres, Yankees, Tigers and Rangers. The elder McCullers also made his Major League debut at 21 years old (1985).

"I was worried someone else was going to tell him, and I called my fiancé to tell her and called my mom and my brothers and I told them, 'Don't tell dad,'" Lance Jr. said. "He finally answered the phone and he said, 'Hey, buddy, how did your bullpen go?' I always call him after my bullpens. I said, 'I didn't throw one.' I said, 'We made it.' He said, 'What do you mean?' I said, 'I got the callup. I'm going to pitch against Oakland on Monday.' He was happy and super excited."

"Words can't describe it," McCullers Sr. said of receiving the phone call from his son. "It was phenomenal. It's something he dreamed about since he was a little kid. He always wanted to follow in my footsteps and play baseball. He loved it. It was a great moment to get the phone call from him and to hear his voice when he told me that he was going to the big leagues."

McCullers has a fastball that sits at 92-98 mph and could hit 100 mph if he was relieving. He was likely targeted for the bullpen had he not refined his changeup last year, forcing himself to throw it at Class A Advanced Lancaster.

"It was going to have to come at the expense of walks, home runs and stats, but I knew I needed to make this pitch happen if I wanted to get to the big leagues as much as I do," McCullers said. "I went months and months throwing it and developing it to become the pitch it has become for me."