JUPITER, Fla. — The Cardinals top prospect and one of the top impact surprises of spring training were among six players sent to Triple-A on the eve of the club’s only off day this spring, a sure sign of a roster taking shape with less than two weeks to play.

Colby Rasmus and Joe Mather will report to the Memphis Redbirds camp on Wednesday, where they will be joined by pitcher Mike Parisi, catcher Mark Johnson, and pitcher Jason Motte. Pitcher Clayton Mortensen was also reassigned to the minor-league camp. Mather, Motte and Parisi were all optioned to Triple-A.

“You can tell by the number of at-bats they got the kind of impression they made,” manager Tony La Russa said of Rasmus and Mather, two prospects who stayed in the thick of the competition for starting jobs all the way to St. Patrick’s Day. Both outfielders had more than 40 plate appearances.

“You don’t get at-bats like that if you’re not impressing someone.”

As a nod to the kind of spring training Rasmus has had, the Cardinals’ top prospect — and widely considered one of the five best prospects in all of baseball — started Monday’s game in center field and led off.

He leaves major-league camp as one of the NL leaders with 12 walks to complement a .273/.467/.515 line and two home runs, seven runs scored and four RBIs.

“I’ll go down and keep playing with a bunch of the guys I’ve been playing with,” Rasmus said, “and try to fight my way back up.”

Where Rasmus entered spring with excess hoopla, Mather came to spring training with little more than a new spot on the 40-man roster. Just a few years ago the Cardinals were about to release the 6-foot-4 slugger, and this past winter it took some staunch advocates in high places to put him on the 40-man roster and protect him from other team’s. Mather repaid his fans with a .526 slugging percentage in spring, the third best among outfielders behind opening day center fielder Rick Ankiel and surging Rule-5 pick Brian Barton.

Mather hit .289 with a .386 on-base percentage and six extra-base hits and four RBIs.

“I feel like I did what I could, did what I had to do,” Mather said. “Just put myself in the picture. I finally played in front of Tony.”

Parisi, a righthander, may be even more in the picture than the other two. While the Cardinals do have several major-league starters they are banking on returning in the first five weeks, Parisi threw himself into the mix as the first starter ready from Triple-A. The righthander drew raves for his throw Monday, as he alternated innings with newcomer Kyle Lohse in a simulated game.

Pitching coach Dave Duncan said Parisi “is going to pitch in the major leagues because he’s got major-league stuff. Very impressive.”

Motte, a catcher turned fireball reliever, had been in the mix for the one suddenly open bullpen spot. His departure leaves three still competing: favorite Kelvin Jimenez, upstart Kyle McClellan and Chris Perez.

The assignments give the clearest snapshot yet of the 13 position players the Cardinals will bring north, and the outfielders who will open the season with the team — including Skip Schumaker, Ryan Ludwick, and Barton, and infielder Brendan Ryan.

The roster moves were made Monday because innings to pitch and playing time will become sparse as the Cardinals devote more of the games to the regulars.

“All of these guys had real solid camps,” general manager John Mozeliak said. ”Not only did they open up the eyes on our major-league staff but they reinforced how a lot of us felt about them and why we brought them in the camp. … It wouldn’t surprise me if I saw either one of them (this season) in St. Louis.”

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