The Diamondbacks, now 24 games off the pace in the National League's West Division, have lost seven straight games as they head to Queens, including three straight at Philadelphia in a series that concluded Thursday night.
Miguel Montero hit his third home run of the season and finished with two RBI for the Diamondbacks.
Joe Saunders made his first start with the D'Backs after he was traded to Arizona in the Dan Haren deal on Sunday. Saunders was charged with two runs on nine hits and four strikeouts over seven innings.
Cody Ransom drew a one-out walk in the bottom of the 11th off of Esmerling Vasquez (1-4) and moved to second base when Carlos Ruiz singled. Valdez then stepped to the plate and laced a base hit to center field. The throw from Chris Young partially hit the pitcher's mound, which helped Ransom slide safely into home plate as the Phillies celebrated the victory.
Arizona has won just 13 of 49 road games so far in 2010.
He earned a 13-2 victory in the initial matchup on July 19 in Phoenix, allowing four hits and a run in five innings to notch his fifth victory of the season.
One start since ended in loss No. 8 for Kennedy, who was touched for six hits and four runs in 6 2/3 innings of a 10-4 defeat against San Francisco.
He is 2-5 in 10 road starts with a 4.70 earned run average.
For the Mets, 6-foot-7 right-hander Mike Pelfrey can erase the bad taste from the aforementioned July 19 game.
The 26-year-old was raked for seven hits and six runs while retiring only four batters and walking two against the Diamondbacks, who handed him his third straight loss since starting the year at 10-2.
Pelfrey, the ninth overall draft selection in 2005, hasn't won since a 5-2 interleague defeat of Minnesota on June 25.
He is winless in six career matchups with Arizona, going 0-5 with a 6.75 ERA. However, he is 6-2 in 11 home starts in 2010.
On Thursday, rookie Ike Davis hit a three-run homer and knuckleballer R.A. Dickey went 8 1/3 innings as the Mets earned a 4-0 win over the St. Louis Cardinals in the rubber match of a three-game home set.
Dickey (7-4), who was working on three days rest after exiting his last start on Sunday with a left leg injury after 5 2/3 innings, was sensational. He yielded only four hits, finishing with two strikeouts and a pair of walks. Francisco Rodriguez retired the final two batters in the ninth to pick up his 22nd save of the season.
The Diamondbacks swept a three-game set at home over the Mets from July 19-21 and have won seven of the last eight in the series.