The Cardinals would prefer to keep first-year pro Michael Wacha in the minors for a majority of this season, but a rash of injuries to starters has left them with no choice and one dazzling pitching prospect.

He is their best available option.

Only 11 months, 29 days removed from his final pitch as Texas A&M’s ace, Wacha will make his major-league debut Thursday as a starter for the Cardinals at Busch Stadium. The Cardinals announced Tuesday that Wacha, 21, will join the team today and be added to the roster Thursday afternoon. The 19th overall pick in last summer’s draft will become the first pitcher in 25 years for the Cardinals to jump from the draft to his debut in less than 12 months.

“People have to keep in mind that a year ago this kid was in the collegiate playoffs,” manager Mike Matheny said. “That’s a rather drastic jump. But he’s earned it. He’s proved that he’s ready. … What we do know is there is great potential there. We also know this is a situation where we need help. We tried to let him work down there without him being the first guy we had to go to, but right now we need somebody to come up.

“Whoever our best guy is needs to be the one.”

Wacha is clearly the “best guy.” The righty is 4-0 with a 2.05 ERA in nine starts and 52 2/3 innings for the Cardinals’ Class AAA Memphis affiliate. His ERA leads the Pacific Coast League and it comes immediately following spring training when he did not allow an earned run during major-league exhibitions nor in minor-league camp.

The 6-foot-6 righty with a devilish changeup impressed coaches and teammates with his command of several pitches and the downward, slicing angle with which he was able to drive pitches into the lower part of the zone. Catcher Yadier Molina volunteered after Wacha’s appearance against the Mets in a Grapefruit League game that the prospect’s stuff was already good enough to succeed in the majors.

The Cardinals, however, planned to be protective of their prized prospect.

The Cardinals closely monitored Wacha’s innings last season by using him as a reliever and scheduling his appearances as he climbed to Class AA. General manager John Mozeliak expressed reluctance to promote Wacha less than two weeks ago because this season was the righty’s first exposure to a five-day rotation. Mozeliak said that Wacha had major-league stuff but the team was hesitant to expose him to major-league demands.

“There are a lot of questions about how this is going to look like as far as usage. Will he be here the entire season? These are questions we don’t have answers for at this time,” Mozeliak said Tuesday. “We’re going to be very cognizant of pitch counts and innings. When we think about total innings, we want to make sure we’re careful of his usage. If there are times when we can protect him, we will do so. We want to make sure that we understand that we should do what’s best for him as well as give the club some protection.”

Mozeliak convened a dinner with Cardinals coaches Monday evening in Kansas City to discuss their options for Thursday’s open start, but also to establish some parameters for promoting Wacha. The team also considered moving Joe Kelly or Seth Maness from the bullpen for the start but decided Wacha was the most “seamless” decision. Mozeliak wanted Matheny, pitching coach Derek Lilliquist and other members of the staff to explore ways they could monitor Wacha’s workload.

If he remains for the rest of the season, Wacha could make 22 starts, Mozeliak said. At five innings a start, that would put his total innings this season around 160. That would “work in terms of not putting him in danger,” Mozeliak said. To control his innings, the Cardinals will consider skipping starts or removing him from games at an unadvertised pitch count. Mozeliak doesn’t want to publicize the specifics because the staff wants flexibility.

The GM said the team would not consider using off days to skip one pitcher because he didn’t want to “reinvent the wheel for one person.” The preferred plan is to have Jake Westbrook and Chris Carpenter return from the disabled list and prospect Carlos Martinez ready to fill in if one of the three rookies in the rotation needs a break.