Cardinals manager Mike Matheny had a blunt description of the task facing center fielder Peter Bourjos as he tries to get more from his swing with what could be a stretch of fewer at-bats.

“Extremely difficult,” Matheny said. “It’s something we’re very well aware of. … The assignment for us is to be honest, as open with him as possible. ‘We’ve got to get you going, but we’ve got to get our club going, too. If we have an opportunity to bring some player in who can get us a jolt we’re going to make a shot at it. Stay ready.’”

The player Matheny referenced is rookie Randal Grichuk, a righthanded hitter like Bourjos who got the start in center field Tuesday. The start was a first for Grichuk in the majors, and he singled in his first at-bat for his first big-league hit. It was a start that Bourjos or Jon Jay would have had had Grichuk not been promoted Monday, and Matheny said he made it clear to Bourjos what Grichuk’s presence could mean for his playing time.

Bourjos, 27, has started once in the past week.

He’s caught in a zero-for-16 spiral that has lowered his average to .160 and chewed into the club’s planned playing time for him.

“Look at an oh-for-15 over a two-week span or a week span and that seems pretty bad but realistically some of those guys are getting the same number of at-bats in three days,” Bourjos said. “It’s tough to get in a rhythm. Obviously the offense hasn’t been producing like we know it can. So obviously it has to take someone’s playing time to get that rhythm going, and it’s been me so far.”

Even with the lack of recent starts, Bourjos has the most this season, 14 to Jay’s 13. The Cardinals acquired Bourjos and Grichuk from the Los Angeles Angels this past winter in exchange for David Freese and reliever Fernando Salas. The Cardinals’ intent was to utilize Bourjos regularly, even every day, as a defensive whiz with offensive potential and speed. Team-wide offensive struggles have magnified his and others.

General manager John Mozeliak suggested that Grichuk’s promotion could be short-term if the prospect isn’t getting regular playing time. Bourjos said he “truthfully has no idea” what opportunity he’ll have. He, in the meantime, said his swing feels good, but he’s unsure how to prove it or show it without playing time.

“That’s a good question,” he said. “I don’t know. I think you try to go in there every at-bat and not change too much when you’re in the lineup. You’ve got to stay consistent and (do) your work in the cage. I know in the past I’ve been in and out of the lineup and I tried to change things. That’s worse.”

GARCIA, MOTTE OFF TO MINORS

Both Jason Motte and lefty Jaime Garcia will begin their rehab assignments this weekend with Class AA Springfield. Garcia, who is recovering from shoulder surgery, is scheduled to start Sunday. Motte, the former closer coming back from elbow reconstruction, is likely to throw at some point this weekend as well, though the Cardinals do not have an official start date for his rehab assignment. Pitchers are allowed 30 days in the minors on a rehab assignment, and the clock starts with their first appearance.

Garcia advanced to the rehab assignment with an appearance of more than 60 pitches against an extended-rehab group at Jupiter, Fla., on Tuesday.

MINDING MANESS

Sinkerball pitcher Seth Maness continues to search for consistency and the strike zone, and now the Cardinals are on the lookout for his confidence as well. The righty took his second loss of the season Monday when he allowed two runs on two hits in the 12th inning. Maness has allowed at least a run in four of his past five appearances, and seven of his nine appearances have come in losses. The righty the Cardinals could count on to induce a double play has had that skill reverse on him, with an increasing amount of fly balls this season.

Matheny has altered Maness’ assignments — sometimes long relief, sometimes with men on base. That approach will continue.

“We’ve got to get him in there in situations where he can have success,” Matheny explained. “It’s really not anything more complicated than that. … We’d like to see Seth have a couple of outings where he comes in, pounds the bottom of the zone and trusts that sinker and see some ground balls. Gets a little confidence going and you’ll see a world of difference.”

Matheny insisted that the command troubles Maness has had with his signature pitch are not related to any physical ailments. Maness’ mechanics have been picked apart as well.

“We need him being a little more confident in himself,” the manager said.

EXTRA BASES

Matt Adams, who doubled in his first at-bat Tuesday, has gone back to wearing a brace on his right elbow, where he has dealt with bone chips and hyperextension in recent seasons. Adams said he intends to wear the brace, which he started using last fall, for the rest of the season. “It’s going to be a thing to help my elbow not get over-extended,” Adams said. “Over the long season, it’s smart to put it back on and make sure that elbow stays 100 percent.” … Kevin Siegrist (back tightness) was unavailable the previous two days but cleared for work Tuesday. … Shortstop Aledmys Diaz was placed on the Class AA seven-day disabled list with a back strain. He’s expected to miss a couple of weeks. Through 20 games, the Cuban infielder has hit .307 with three homers and a .520 slugging percentage.

Derrick Goold covers the Cardinals and Major League Baseball for The Post-Dispatch. Follow him on Twitter at @dgoold or on Facebook at Facebook.com/BirdLandPD

Derrick Goold is the lead Cardinals beat writer for the St. Louis Post-Dispatch and current president of the Baseball Writers' Association of America.