LAS VEGAS - Mets righthander Jenrry Mejia took the mound Sunday unaware of Jonathon Niese's date with an MRI machine. But the results of Niese's left elbow exam in New York Monday might determine whether Mejia begins the year in the starting rotation.

Mejia struggled with his command in a 6-3 loss to the Cubs Sunday, allowing three runs (two earned), four hits, two walks and a hit batsman in 21/3 innings.

Mets manager Terry Collins said Mejia remains in the mix, especially if Niese's elbow discomfort leads to an extended absence. Mejia said he wants to keep his focus on his own performances, regardless of Niese's status.

"My arm feels great," Mejia said. "I can't complain."

Prospect Rafael Montero is not a candidate to fill a potential vacancy in the starting rotation. Collins said the Mets are considering him for a spot in the bullpen.

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Short leash?

Wilmer Flores will get more starts at shortstop, Collins said, perhaps an indication of waning support for Ruben Tejada.

"He's not done playing out there," Collins said. "We've got to have somebody ready."

During the Mets' 10-4 win over the Cardinals in Jupiter, Fla., Tejada booted a routine ball in the third inning for his fourth error in his last seven games -- all on routine plays.

Tejada ended the inning by starting a double play but finished 0-for-4 to drop his average to .091 (2-for-22).

Extra bases

Juan Lagares turned in two sparkling defensive plays against the Cubs. The centerfielder took away a potential hit with a diving catch before throwing out a runner at home plate . . . Ike Davis (calf) was 2-for-6 in minor-league at-bats. He did not run the bases, though he ran during conditioning drills. Lucas Duda (hamstring) was given the day off.