HOUSTON -- Outfielder Lucas Duda and pitcher Frank Francisco were ailing and unavailable on Monday.

Duda was taken out of the starting lineup because of flu-like symptoms.

"He's sick enough that he can't play," Collins said. "He feels terrible today."

Collins said that Duda stayed in the clubhouse with a hooded sweatshirt, another jacket on top of that and a heating pad. Collins had Duda remain in the clubhouse to get fluids and perhaps be available for late-inning pinch-hit duties.

"But I'd rather do that than take him out of the game in the second inning and lose two guys," Collins said.

In the seventh inning, with the Mets trailing by two runs and runners on second and third with two outs, Duda pinch-hit and earned a walk to load the bases ahead of Kirk Nieuwenhuis' two-run single.

Francisco is suffering from an upper hamstring issue and is day to day.

"We didn't want to aggravate it," Collins said. "It's bothered him the last couple of days."

Pelfrey to undergo surgery on Tuesday

HOUSTON -- As expected, Mike Pelfrey will undergo Tommy John surgery, the Mets announced Monday. Pelfrey will have the surgery on Tuesday and likely miss at least one year.

Pelfrey, 28, pitched in 19 2/3 innings this season, compiling a 2.29 ERA with no decision in three starts.

He was examined by Dr. James Andrews on Monday and it was determined that the partial tear would require season-ending surgery.

Pelfrey was 7-13 with a 4.74 ERA last season for the Mets, which followed a career-best 15-9 (3.66 ERA) campaign in 2010.

Torres back in center, hits seventh in lineup

HOUSTON -- Andres Torres returned to the Mets' lineup on Monday, playing center field and batting seventh in the 4-3 loss to the Astros. Manager Terry Collins said the lineup decision was two-fold.

"He really hasn't had a lot of at-bats, so I didn't want to mess around with the leadoff spot just yet," Collins said before the game. "You're looking at 25 at-bats in Spring Training. In the five-game rehab, he's probably had 20 more at-bats. That's not a lot to start the season.

"And the one thing he does at the bottom of the order, if they bring in a lefty, we've got a righty that can basically break those lefties up."

Torres, who went 1-for-4 with an RBI in his first game, said the strained calf that sent him to the DL on April 6 was feeling good enough to get back out in center field.

"It feels great," said Torres, who hit .263 (5-for-19) with four runs scored and three stolen bases in five rehab games. "It feels real good, I made some running catches, stole some bases, and it felt great."

To make room on the roster, the Mets optioned Zach Lutz to Triple-A Buffalo of the International League after Sunday's game.

Nieuwenhuis remains atop the order

HOUSTON -- Rookie Kirk Nieuwenhuis remained in the leadoff spot of the Mets' batting order on Monday, but with the return of center fielder Andres Torres, did move to left field.

Nieuwenhuis said he doesn't hold any expectations of remaining at the top of the batting order.

"That's tonight," he said before going 2-for-4 with two RBIs in a 4-3 loss. "I don't know about next week. That's just for tonight, we'll see."

Collins said his decision to keep Nieuwenhuis (.316, two homers, eight RBIs) at the top of the order was an easy one.

"Forty-five hits in three games," Collins said, of a recent tear that included three hits on Sunday, Nieuwenhuis' third three-hit day this season.

His move to left field was equally a no-brainer.

"We looked up a lot of numbers, Andres Torres is a pretty good center fielder," Collins said.

"Kirk played the corners in Spring Training. He hasn't during the season, but he's athletic enough, he can play out there."

Nieuwenhuis said he feels prepared for the move.

"BP helps," he said. "But it is a big difference, the spin of the ball off the bat. The visuals, it's different. But BP helps a lot."