The oft-injured Alou left spring training camp Wednesday and was sent back to New York for tests on his sore right groin. He is scheduled to have the surgery on Thursday.
Alou is expected to resume baseball activities in four to six weeks, making it likely he'll be sidelined for at least most of April.
MLB.com reported on Wednesday that Alou was injured in a spring game against Atlanta on Monday.
"After the game, he felt it more and more," Mets general manager Omar Minaya said, according to the Web site. "He felt some more discomfort [Tuesday], and [Wednesday] he also felt discomfort."
The 41-year-old Alou batted .341 with 13 home runs and 49 RBIs in 87 games last season, when he missed a long stretch of games with a quadriceps injury.
"We came into the season with a track record of Alou, knowing that Alou was going to give us a certain amount of games," Minaya said, according to MLB.com. "We felt that the number of games he was going to give us would be very productive games, and I think that bore out last year."
Now, six of New York's eight regulars are sidelined, plus one member of the rotation -- and it's still early in spring training.
Remember all that excitement and bravado after Johan Santana was acquired from Minnesota last month? With the best pitcher in baseball suddenly on their side, the star-studded Mets were supposed to be NL favorites.
But age is becoming a major question mark for this team, which might be too injury-prone to win.
"I myself have never seen so many injuries at one time," Minaya said. "The good thing is it's early in camp."
Alou's injury overshadowed a strong outing by Santana in his second start this spring. The two-time Cy Young Award winner showed off his baffling changeup, striking out four in three innings of New York's 3-2 victory over the Los Angeles Dodgers.
"I'm fine. I'm right where I'm supposed to be," said Santana, who threw 54 pitches.
The left-hander was matched up with fellow ace Brad Penny, who tossed three effective innings in his second start against the Mets in four days.
The Dodgers also lost a key player to injury when second baseman Jeff Kent was scratched from the lineup with a strained hamstring. He could miss at least a week.
"He's a veteran and with those guys when they get to this stage of their career you give them a little more leeway, a little more time," manager Joe Torre said.
Besides Alou, the Mets are missing first baseman Carlos Delgado (sore right hip), right fielder Ryan Church (concussion) and catcher Brian Schneider (hamstring). Center fielder Carlos Beltran (both knees), second baseman Luis Castillo (right knee) and pitcher Orlando Hernandez (toe) are yet to play in a spring training game because they're still rehabbing from offseason surgery.
In the bullpen, Duaner Sanchez reported soreness after his outing Friday against St. Louis, his first game since injuring his shoulder in a July 2006 taxi accident. He might not be ready to start the season.
"There is not going to be a set schedule with Duaner," Minaya said.
Then there's the bench.
Ruben Gotay hurt his right ankle sliding into home plate Sunday, though tests didn't show a fracture. Marlon Anderson bruised his sternum Saturday in a collision with Church.
Damion Easley (ankle), Endy Chavez (ankle and hamstring) and Jose Valentin (knee) haven't played in a game yet because they're still recovering from previous injuries.
"Meet the Mets, greet the Mets" goes the happy old song at Shea Stadium.
How about, "Treat the Mets," with whatever is left in the training room?
"There's a difference between being banged-up and injured. We're not injured, we're banged-up," third baseman David Wright said before learning Alou would need surgery. "Opening Day, we're running out this lineup, then that's when it's time to panic a little bit. But right now, second week of spring training, these guys are fine."
Hernandez threw a 45-pitch bullpen session and felt good in his new windup -- without the high leg kick. The altered motion helps El Duque keep his back foot flat on the rubber during his delivery, which eases stress on his surgically repaired toe. Church said he felt awful Wednesday and the team told him to rest. He had been encouraged Tuesday when he worked out in a pool and felt much better. Delgado hit off a tee Wednesday. Mets RHP
Pedro Martinez is scheduled to make his first spring training start Thursday night against Washington. Dodgers RHP Chan Ho Park struck out three in two scoreless innings.
Information from The Associated Press was used in this report.