Rosario hit a tiebreaking homer in the seventh inning, and the Rockies beat New York 5-2 on Wednesday night for their seventh straight victory at Citi Field.
Rosario also had a sacrifice fly, and DJ LeMahieu drove in a run with a squeeze bunt to help the Rockies win their fourth in a row and eighth in 10 games. Colorado will go for a four-game sweep on Thursday, a feat the Rockies accomplished during their previous visit to Queens in April 2011.
"It seemed like from what I read, though, in previous years we didn't like it too much. But I guess we're starting to like it a little bit more," Colorado manager Jim Tracy said. "I'm just very pleased with the way our team's playing, whether it's in New York, Chicago, Coors Field, San Francisco, Los Angeles."
Prized rookie Matt Harvey had another impressive outing for New York, striking out nine over six innings of three-hit ball. But the slumping Mets (57-67) lost their fourth straight and fell to 11-27 since the All-Star break, leaving them 10 games below .500 for the first time since finishing 70-92 in 2009. They have been held to two runs or fewer in five consecutive games for the first time since September 2003.
With two runners on in the ninth, Ronny Cedeno flied out to the left-field warning track to end it.
"I feel like there's more that I could've done and more I could've helped the team with -- like going eight innings and not giving up a run would've been nice," Harvey said.
Before the game, the Mets placed struggling ace Johan Santana on the 15-day disabled list with inflammation in his lower back and said he isn't expected to pitch again in 2012. Doctors recommended rest, but not surgery.
The 33-year-old Santana, who missed last season following major shoulder surgery in September 2010, went 0-5 with a 15.63 ERA in his final five starts.
Jeff Francis allowed one run and five hits in a five-inning start, though not all the news was good for the injury-ravaged Rockies. Center fielder Dexter Fowler left in the fourth with a sprained right ankle after twisting it while cruising into second base.
X-rays were negative, but Tracy said Fowler is likely to miss at least a couple of days.
"It doesn't feel good right now," said Fowler, who hobbled around the clubhouse on crutches. "Probably a couple days and re-evaluate it."
New York cut it to 3-2 in the eighth. Scott Hairston, who had a sacrifice fly in the first, hit a two-out double off winner Carlos Torres (2-1) and scored on Ike Davis' single against Matt Reynolds. Will Harris, however, struck out pinch hitter Jordany Valdespin to preserve the lead.
Last in the NL West, Colorado added a pair of insurance runs in the ninth, aided by catcher Kelly Shoppach's throwing error and reliever Frank Francisco's balk. Chris Nelson had an RBI single, and pinch hitter Josh Rutledge delivered a run-scoring double.
"It's another night, just like last night, where we did a great job with situational at-bats," Tracy said. "We hung in, and when we needed to execute offensively to produce some runs, we did so."
Rafael Betancourt worked a scoreless ninth for his 25th save in 30 attempts.
"We're certainly playing better baseball," Francis said. "As a pitching staff as a whole, we're keeping our team in games a lot more."
Rosario connected off Ramon Ramirez (2-3) leading off the seventh. The rookie catcher lofted a high fly into the left-field corner for his 20th home run to give the Rockies a 2-1 lead.
Tyler Colvin then doubled, moved up on a groundout and scored on LeMahieu's nicely executed bunt.
"We've got a lot to work on. We're a young ballclub and we've got to take these games seriously, and I know these games are going to help us out," Colvin said.
Torres yielded one hit in 2 2/3 innings before a sparse crowd of 22,204.
Harvey has 43 strikeouts through his first six major league games. The Mets said that broke Nolan Ryan's club record of 42, according to research by the Elias Sports Bureau.
Harvey also went 2 for 2 with a pair of singles and is batting .455 (5 for 11) with two doubles and two RBIs.
"Extremely impressed tonight with the way he handled himself, pitching out of trouble, electric stuff again," Mets manager Terry Collins said. "He asks all the right questions. He pays attention. He's the first guy on the bench before the game starts. He's there throughout the game. He listens. He wants to learn. He truly is living up to that, 'I'm not here to be one of the guys. I'm here to be THE guy.' And we think we've got some more coming. During an absolutely brutal stretch, there's been a bright spot, and he's been one of them."
Rosario tied Charles Johnson's record for homers by a Rockies catcher, set in 2003. He leads NL rookies in home runs. ... Tracy said RHP Jhoulys Chacin, a winner Tuesday night in his return from the disabled list, will start again Sunday against the Cubs. ... New York is 2-13 at home since the All-Star break.