TOKYO -- A's top prospect Jesus Luzardo is no longer an Opening Day rotation option.

The much-hyped lefty has been shut down with a muscle strain in his left shoulder for four to six weeks, A's general manager David Forst announced prior to Thursday's Opening Series finale with the Mariners in Tokyo.

It's an unfortunate setback for the 21-year-old Luzardo, who had hoped to push his way into the rotation despite minimal experience at the Triple-A level.

"He's disappointed," Forst said, "but he knows he just has to take his time with this."

The A's have three remaining candidates for two spots, and the out-of-options Frankie Montas and Aaron Brooks are seemingly favored over Chris Bassitt.

Luzardo likely won't come into play until June at the earliest. That's when the A's also hope to have starters Jharel Cotton and A.J. Puk back from Tommy John surgery.

Oakland lost Puk -- like Luzardo, a highly touted, hard-throwing lefty -- around this same time a year ago amid an eerily similar situation.

"You have to understand who we have coming back later on," A's manager Bob Melvin said. "And you look at the potential of the rotation next year, it looks really good. We just need to hold the fort down for a little while here, and we feel like we have the guys to do it."

Forst said Luzardo has experienced some discomfort for a while now, but it worsened after he threw a side session in Arizona on Monday, leading him to speak up about it.

"You have to make sure it heals before he gets back there and throws," Forst said. "It's something we have to be patient with, because if we get him out there too soon, it's just like any muscle strain. You run the risk of doing more damage."

Luzardo was superb this spring, allowing a lone earned run with 15 strikeouts over 9 2/3 innings. He was recently named the game's top left-handed pitching prospect by MLB Pipeline after climbing three levels in the Minors in 2018. He compiled a 10-5 record and 2.88 ERA with 129 strikeouts in 109 1/3 innings.

"He was still pitching for a spot and I don't know that anybody performed better this spring for us, but we know what we have down the road " Melvin said. "It will just take a little bit more time at this point. You rest this thing up and he's healthy when he comes back, and we hopefully have him for the prime of the season."

The young southpaw had Tommy John surgery three years ago and likely would have been limited in innings, anyways, this year.

"You have to look at, how many innings was he going to throw this year? This kind of maybe helps us out as far as that goes," Melvin said. "He starts the season with us and never pitched in September as well. How would he be down the stretch? I think sometimes things happen for a reason. You hate to see anybody get hurt. We hope that this is a sprain and nothing more than that."

"He was a candidate to make this team," Forst said. "And our plan was to have him at the Bay Bridge Series and have him start one of those games and then make the decision. So obviously it now comes down to other guys for the fourth and fifth spot."

Montas will pair with Brett Anderson in the Sunday's Bay Bridge Series opener. Relievers will handle the second game, and Brooks and Bassitt will be on display in the finale.