The 35-year-old felt tightness while driving home from Legends Field on Thursday and received treatment for about five hours on Friday.
"It's pretty sad when you get to the age when you make a sudden move and stuff happens," Pettitte said. "It's kind of frustrating."
Girardi said that he does not believe the skipped start is a major cause for concern.
"Back spasms can happen to anyone," Girardi said. "You worry if there's a guy with a cranky shoulder or a cranky arm. There's probably been four or five guys [on the Yankees] who have had back spasms all spring."
Pettitte missed a start on March 12 at St. Petersburg with tightness in his left forearm. He was also sidelined last spring after feeling back spasms in a similar location, suffered while performing squatting exercises.
"If you're going to have this kind of stuff, now is the time to get it out of the way," Yankees general manager Brian Cashman said.
Pettitte's final Grapefruit League appearance is tentatively scheduled to come on Thursday against the Pirates at Legends Field. Girardi said that Pettitte should be able to get to 85 or 90 pitches in that start.
"I just want to get well and hopefully be able to make all my starts during the season," Pettitte said.
In the event Pettitte is not ready to face the Blue Jays on April 2, Girardi said the Yankees would consider Joba Chamberlain a "legitimate candidate" for the start, even though he was informed earlier in the week that he would begin the season in the bullpen.
Chamberlain is not so far removed from starting -- he has only made one appearance since the decision was delivered Wednesday -- that he would not conceivably be available to start.
Girardi said that the Yankees would not want to bounce Chamberlain between starting and relieving, so more likely, the Yankees would entertain a long reliever for that start -- a job that Kei Igawa, Jeff Karstens and Darrell Rasner are all competing for.
Rasner threw four shutout innings against the Rays on Friday, Igawa was slated to pitch against the Blue Jays on Saturday and Karstens will start against the Pirates on Sunday.
Pettitte was 15-9 with a 4.05 ERA in 36 games (34 starts) for New York last season, returning on a one-year, $16 million deal for the 2008 season.