Nava connected on a fastball from Joe Blanton in the second inning and shortly after being called up from Triple-A Pawtucket. Kevin Kouzmanoff connected on the first pitch he saw Sept. 2, 2006, for Cleveland against Texas, according to STATS LLC.
Only four players in big league history have hit grand slams in their first at-bat.
Daisuke Matsuzaka was a late scratch and placed on the 15-day disabled list with a strained forearm following his pregame bullpen warmup.
Emergency starter Scott Atchison, recalled from Triple-A Pawtucket on Friday, worked three innings, allowing two runs and two hits in his first major league start.
With his parents watching from the stands - and father operating a video camera - Nava came to bat with the Red Sox trailing 2-1 in the second. In one pitch he became the 10th player in Red Sox history to homer in his first at-bat, and just the second to hit a grand slam. Rip Repulski hit a slam on May 10, 1960.
He came up with the bases loaded again an inning later and received a standing ovation. But he struck out on a check swing with the crowd on its feet.
J.D. Drew homered and had three hits for the Red Sox, who beat the Phillies 12-2 on the opener of a three-game series on Friday night. Nava also doubled as Boston amassed 16 hits.
Manny Delcarmen (2-2), the second of five pitchers, worked two scoreless innings for the win.
Brian Schneider had three hits and drove in both of Philadelphia's runs. The Phillies are 7-16 in their last 23 games.
The Red Sox pounded Blanton (1-5) for nine runs and 13 hits in four innings. But, he didn't fare as badly as Friday's starter, Jamie Moyer, who had the worst start in his 24 seasons, allowing nine runs and nine hits in one-plus inning.
Boston broke it open with five runs in the second and three in the third.
With the Red Sox trailing 2-1 in the second after Drew's homer, they loaded the bases before Nava hit his slam into the Boston bullpen in right.
Marco Scutaro had a two-run single to make it 7-2 in the third. Dustin Pedroia added an RBI single.
Matsuzaka had warmed up and walked to the dugout about 15 minutes before the scheduled start.
Just ahead of the national anthem, Atchison started warming up quickly in the bullpen. When it was over, he completed his warmups before walking to the dugout with pitching coach John Farrell along side.
The club announced the scratch and roster move just before the Red Sox took the field.
NOTES: Outfielder Josh Reddick was optioned to Pawtucket. ... Every Red Sox starter had a hit by the fourth inning. ... Blanton has allowed five or more runs in each of his last four starts. ... Kevin Youkilis was hit on the right arm by a pitch in the sixth inning, the 65th time in his career to move ahead of Jim Rice for second all-time in club history. Mo Vaughn leads with 71. Youkilis then left the game in the top of the seventh. ... Boston recalled LHP Dustin Richardson from Pawtucket before the game. He jogged out to the bullpen before the fifth inning and warmed up briefly. ... The Red Sox have won the last six games in Fenway Park against Philadelphia.
