TEMPE, Ariz. -- It wasn’t unexpected, but former Angels top prospect Jo Adell will open the season in the Minor Leagues, as he was one of four players optioned on Sunday night. Infielder Matt Thaiss, reliever Kyle Keller and left-hander José Suarez were also optioned.

Angels manager Joe Maddon made it clear in December that he believed Adell needed more seasoning in the Minors. The club acquired veteran Dexter Fowler to be the starting right fielder while also signing non-roster invitees Juan Lagares and Jon Jay to boost its outfield depth. Maddon liked what he saw from Adell this spring, but the plan was always for him to open the season in the Minors.

“I kind of recapped it to him [Sunday] when I spoke to him, but I think he’s made a lot of progress actually,” Maddon said on Monday. “At the plate, his swing is so much shorter, his strides so much more under control. Because his foot is getting down sooner, the bat's not moving around and wrapping as much. He looks so much better on defense, he's been working really hard.”

Adell, 21, was the Angels' highest-regarded prospect since Mike Trout entering last season, but he struggled in his first taste of the Majors. He batted .161/.212/.266 with three homers and seven RBIs in 38 games, with 55 strikeouts in 132 plate appearances. He also struggled on defense, notably allowing a ball to hit off his glove and over the fence for what was ruled a four-base error.

But Adell worked hard in the offseason to improve and has looked much better at the plate this spring, batting .263/.440/.632 with two homers and six RBIs in 15 games. Adell, though, has still had some issues defensively, including missing a fly ball in center field in Friday’s game against the Royals.

"I know he's had a couple [mistakes] like the fly ball in the sun, he's slipped and fallen a couple of times, but, again, he's gaining more information there,” Maddon said. “And the same things on the bases. He's a sponge. He listens really well.”

Adell will open the year at the club’s alternate site to continue to work on his defense, but he should take over in right field again at some point this year if he improves. Maddon explained that it doesn’t make sense to have him sit on the bench in the Majors right now.

“He just needs to play baseball,” Maddon said. “He needs nine innings of baseball on a consistent basis. He needs to take all the stuff he's learned and put it into a game. That's the next step for him. Because I have a lot of faith in him and I like what he's doing. He needs to play and doesn't need to be on anybody's bench right now."

Thaiss, 25, has been working on his versatility, playing at second base and in the outfield in addition to both corner infield spots. There was no clear path to him making the Opening Day roster with the club set at both corners with Jared Walsh, Albert Pujols and Anthony Rendon. He also struggled offensively, going 2-for-17 this spring.

Keller had a chance to crack the bullpen but posted a 6.14 ERA in six appearances with nine strikeouts and three walks in 7 1/3 innings. The 27-year-old has pitched 13 innings in the Majors over the past two seasons, including 2 1/3 innings with a 7.71 ERA with the Angels last year.

Suarez will get stretched out to start at the club's alternate site, giving the Angels some rotation depth. He has struggled in the Majors over the past two seasons with a 7.99 ERA in 83 1/3 innings. But he's still just 23 and will get time to work on things in the Minors.