Mets starting pitcher Jonathon Niese, center, is greeted by teammates...

Mets starting pitcher Jonathon Niese, center, is greeted by teammates after he was relieved during the eighth inning of a baseball game against the Los Angeles Angels on Saturday, April 12, 2014, in Anaheim, Calif. Credit: AP / Jae C. Hong

Jonathon Niese insisted that he is not injured.

The lefthander said Sunday that if necessary, he could keep pitching through the lingering weakness in his left shoulder, a condition he's coped with for more than a month.

"This is just a rest period," he said after the Mets' 8-4 win over the Rangers. "I'm not hurt."

Nevertheless, the Mets placed their best pitcher on the 15-day disabled list with what they termed a strained left shoulder. With his history of shoulder woes, the Mets wanted to play it safe with Niese, whom they expect to be ready to pitch in San Diego after the All-Star break.

"This guy's pitched very well and you've got to be careful because we went through it last year, we went through it in spring training," manager Terry Collins said. "So we're just making sure we don't cause a huge problem."

On a day in which they scored early and often, the Mets should have been in position to enjoy that rare combination. Anthony Recker hit a three-run homer in a five-run first inning, Kirk Nieuwenhuis added a solo shot and Zack Wheeler (4-8) held the Rangers to one run in 61/3 innings. The Mets' much-maligned offense piled on as they took two of three from Texas.

"It's nice to get the five-run lead," Daniel Murphy said. "But for the lineup to add on runs, that's the biggest thing."

Instead, Niese's immediate future took center stage. Though Niese has not complained of pain or discomfort, Collins said he raised red flags in his last few starts with a dip in velocity and a noticeable drop-off in the sharpness of his secondary pitches.

Said Collins: "You don't need radar guns to tell you there's something going on here."

In 17 starts, Niese is 5-4 with a 2.96 ERA. If general manager Sandy Alderson chose to put him on the trade block, Niese would be considered the club's most attractive chip leading up to the deadline.

To Collins, Niese has been worthy of consideration for the NL All-Star team, but his first half will end on the shelf. Righthander Dillon Gee, nearing a return from a strained lateral muscle, will make Niese's next scheduled start Wednesday.

"[Gee] felt great," Collins said Sunday. "He's all fired up about it. So we're excited to get him out there."

Reliever Buddy Carlyle was summoned from Triple-A Las Vegas to fill the spot of Niese, who is expected to resume throwing in the next few days.

Niese was forced to cut his start short after 12 pitches Friday when a comebacker left a bruise on his back. But Collins said the Mets already had been discussing whether it would be wise to give him some added rest heading into the second half.

Niese's velocity has steadily declined since last season. His fastball averaged 91.3 mph in April 2013. Last June, that average dipped to 89.1 mph. Said Collins: "There is a time, I think all pitchers go through it, where he's just getting fatigued."

Niese's injury history has pushed the Mets toward acting with caution. He missed two months last season with a partial tear of his rotator cuff and began this season on the disabled list with weakness in the scapula muscle in his left shoulder.

Niese, 27, believes rest should be enough to help the inflammation in his shoulder die down.

"It just seemed like each start, it got progressively more tired and more weak," he said. "I think it's smart and I think it's best for the team if I did get it better."