PORT ST. LUCIE, Fla. -- Kirk Nieuwenhuis sported a heavy limp Monday as he walked through the Mets' clubhouse, but the outfielder believes he managed to avoid the worst. An MRI revealed that Nieuwenhuis has a bone bruise in his left knee but no ligament damage, perhaps allowing him to avoid an extended absence.

"It could be a few days; it could be a week," Nieuwenhuis said. "I've just got to kind of wait it out and see how it responds to the medicine."

Nieuwenhuis injured his knee sliding headfirst into second base during a game against the Marlins on Sunday. Though he is not sure precisely how he hurt himself, Nieuwenhuis believes he banged his knee on Roger Dean Stadium's infield dirt, which he compared to "concrete."

Injuries are nothing new for Nieuwenhuis, who missed the second half of the 2011 season with a torn labrum in his left shoulder, then strained a right oblique muscle last spring, and then suffered a season-ending plantar fascia tear in his right foot in August. The latter ailment came after Nieuwenhuis jumped out to a hot start in his rookie season but faded rapidly at the plate, particularly against left-handed pitching.

"It's part of the game, but it's probably the most frustrating part of the game," Nieuwenhuis said of his injuries. "Thankfully, this isn't serious."

The Mets envision breaking camp this spring with Nieuwenhuis and Collin Cowgill manning two halves of a center-field platoon. But Nieuwenhuis must first prove he is healthy, then that he is right at the plate; prior to his injury, he was 1-for-18 with seven strikeouts in Grapefruit League play.

If Nieuwenhuis does not break camp with the team, defensive standout Matt den Dekker could make the club in his place. More likely, the Mets would use Mike Baxter in a center-field platoon with Cowgill, hand Cowgill the position outright, or put Marlon Byrd in center and Baxter in right.

Collins talks to Johan about taking it slow

PORT ST. LUCIE, Fla. -- A day after Johan Santana surprised most of Mets camp by throwing off a mound well ahead of schedule, manager Terry Collins approached Santana to tell him his aggressiveness was "unnecessary."

"In retrospect, he knows he should have handled it a little bit differently," Collins said. "But he was fine, and he wanted to show everybody he was fine."

Santana appeared visibly agitated in the clubhouse Sunday morning, a day after general manager Sandy Alderson acknowledged his disappointment that Santana did not arrive at Mets camp in better shape. Alderson said Saturday that Santana was roughly 10 days away from taking a mound -- words that Collins believes motivated the left-hander to prove everyone wrong.

Calling Santana's bullpen session an unnecessary risk, the manager was displeased that he jumped so far ahead of his schedule in apparent response to criticism.

"I just said, 'Look, we're going to do this the right way,'" Collins said of his chat with Santana, who is battling left shoulder weakness. "'We're not going to hurt you. We're not going to get you injured. I don't care how upset you are about things, we're going to do this the proper way. April 1 is only a date on the calendar. It doesn't mean anything if you're not ready.'"

Santana is still aiming to start Opening Day for the Mets on April 1. The team hopes he can make his Grapefruit League debut at some point next week, putting him in line for three exhibition starts prior to the regular season.

Hefner, McHugh focusing on what they control

PORT ST. LUCIE, Fla. -- Mets manager Terry Collins has been shy about discussing the starting pitchers on the fringe of his depth chart. Jeremy Hefner is on the list. Collin McHugh is there. So are Jenrry Mejia and Aaron Laffey.

Those pitchers understand the cause-and-effect that faces them at Mets camp. If Johan Santana is not healthy enough to break camp in the rotation, they know, one of them will take his place.

It will probably not be Mejia, who has struggled early in Grapefruit League play and appears to be an unfinished product. Laffey is also a long shot because he is not a member of the 40-man roster.

That leaves Hefner and McHugh, both of whom served as swingmen for the Mets last season. Hefner in particular spent much of the summer in New York, making him the leading candidate to fill in if Santana misses any significant time.

"First and foremost, I want Johan to be healthy," the right-handed Hefner said after pitching three innings of one-run ball Monday against the Braves. "He's a leader of this team. If something happens and he's not able to go, then obviously that would maybe fall on my shoulders. I'm going to be prepared to go out and give it my best whenever my name is called, and whatever happens with Johan will take care of itself."

Hefner learned to be flexible last season, when he made 13 starts -- many of them on short rest or short notice -- and 13 relief appearances as a rookie.

McHugh does not need to be quite so flexible, considering he will break camp as a starting pitcher one way or the other -- either in New York or at Triple-A Las Vegas. Still, he cannot avoid the speculation.

"You like to say you never pay attention to it, but obviously you hear stuff," McHugh said. "I think that the challenge is when you hear the stuff, not letting it change your approach, not letting it change the way you go about your business. Because if you hear, 'Oh, he's in the running for that sixth rotation spot,' and it changes you … that's when you start getting into trouble."

Worth noting

• Right-hander Zack Wheeler will play catch this week with the aim of returning to the mound for a bullpen session Thursday. Wheeler has not pitched since straining a right oblique muscle during batting practice on Feb. 28. The Mets continue to err on the side of caution with their top pitching prospect because, in Collins' words, "he's not going to make the club anyway."

• Shaun Marcum will return to the mound in a "B" game Thursday after missing one turn of the rotation to participate in an arm-strengthening program. Marcum appeared in the Mets' first Grapefruit League game on Feb. 23, but decided afterward that he wanted to extend his long-tossing program rather than immediately appear in another game.