Halladay (1-0) allowed six hits and struck out seven. The reigning NL Cy Young Award winner gave up one run in six innings in the opener against Houston, and avoided a loss when the Phillies rallied in the ninth for a 5-4 win.
The four-time NL East champions are off to a 5-1 start in a season in which nothing less than a second World Series title in four years will be considered a success.
Valdez, filling in for the injured Utley, finished 4-for-4 with three RBIs, two doubles and two infield singles. He's one of six regulars hitting above .300 in the first six games.
Utley, a five-time All-Star second baseman and the team's No. 3 hitter, is out indefinitely with a knee injury. Losing Utley, coupled with the departure of Werth, the former No. 5 hitter, was supposed to weaken Philadelphia's offense.
So far, it hasn't been a problem. The Phillies are averaging 7.2 runs and 12.7 hits per game.
"It's better than I expected," manager Charlie Manuel said. "But at the same time, we played six games. I don't want to talk about it. It's best not to talk about it and let it happen."
"We know he's not here right now," Valdez said of Utley. "Hopefully when he comes back, he's 100 percent. But we have to keep playing. When he gets back, we'll be better."
New York starter Jonathon Niese (0-1) got roughed up by an offense that he dominated last season. The left-hander allowed six runs and eight hits in four innings.
The Mets head to Citi Field for their home opener Friday against Washington after splitting a six-game road trip.
"It's not the way you want to start the season before you go home," Mets manager Terry Collins said.
The Phillies got going with a pair of runs in the third. Valdez reached on an infield single up the middle to start the inning. After Halladay sacrificed, Shane Victorino lined an RBI single to center. Victorino advanced to second on center fielder Angel Pagan's throwing error, and scored when Polanco drove a double to deep left.
Niese nearly escaped further damage by striking out Halladay and Victorino. But Polanco lined a two-run single, putting Philadelphia up 6-0.
Halladay has picked up where he left off last year when he won 21 games in his first season with the Phillies, including a perfect game. He also tossed a no-hitter in his first career postseason start.
The big right-hander has an 0.69 ERA in two outings.
"When you get a lot of runs, it makes it easier to be aggressive," Halladay said.
Halladay pitched out of a jam in the third when the Mets loaded the bases with one out on three singles. He struck out David Wright swinging at a 91 mph cutter, then retired Ike Davis on a grounder to second to end the inning.
Davis snapped his RBI streak. He drove in at least one run in each of the club's first five games. Wright established the team record with at least one RBI in each of the first six games in 2006.
Phillies closer Brad Lidge reiterated he hopes to return by the All-Star break. Lidge has a strained rotator cuff and has been sidelined since spring training. ... The last time the Mets won their first two road series came in 2002 against the Braves and the Cubs. ... The Phillies are 5-1-1 in their past seven home series against the Mets. ... The leadoff batter for both teams reached in each of the first four innings. Philadelphia's leadoff batter reached in all eight. ... A crowd of 45,468 was the 142nd consecutive sellout at Citizens Bank Park, including postseason. ... Jose Reyes had two hits off Halladay, improving to 10-for-22 in his career.