JUPITER, Fla. – Spring training isn’t even over yet and the Mets already have a major hole in their starting rotation.

No. 5 pitcher Steve Trachsel, always counted on to make his starts and chew up innings, will have surgery to repair a herniated disk in his back and is likely to miss a major portion of the season.

“This is not what I expected when I left last week to go to New York for Xrays,” Trachsel said in a statement yesterday. “I thought I would get a shot and be back. There is no doubt in my mind I’ll be pitching again before the end of the season.”

The Mets will first look internally to fill Trachsel’s spot. Who will take his place is the big question. Players who have this type of operation normally miss about three months, said Mets General Manager Omar Minaya, who wouldn’t speculate on how long Trachsel will be out.

Right handers Matt Ginter, Jae Seo, and Aaron Heilman would be the most likely to replace Trachsel as the fifth starter in the Mets rotation.

“The opportunity is there for a few guys,” Mets manager Willie Randolph said. “We’ll see who takes the bull by the horns.”

Trachsel flew to New York over the weekend to have an MRI exam after experiencing discomfort in his lower back following his most recent start. He then headed to California to receive a second opinion. Minaya said yesterday that the MRI had shown evidence of a herniated disk.

Trachsel, 34, went 12-13 with a 4.00 ERA. He has made at least 28 starts each of the last 10 years.

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PIRATES 9, YANKEES 4

Kevin Brown was so concerned with staying on his throwing schedule that he flew back to Florida from his Georgia home only hours after his wife gave birth. After his first few pitches, he probably thought it was a bad idea.

Brown was roughed up for seven runs and eight hits in 4 2/3 innings yesterday by one of the majors’ lightest-hitting teams in the Yankees’ 9-4 loss to Pittsburgh.

Ty Wigginton drove in four runs with a homer and a double, and Rob Mackowiak homered twice – once off Brown – and drove in three runs.

Brown probably wishes he stayed at home with wife Candace, who gave birth to the couple’s fourth child on Tuesday. He flew into nearby Sarasota yesterday morning, only to be greeted by 25-mph winds that contributed to a couple of the Pirates’ four homers. All but two of the Pirates’ 12 hits went for extra bases.

“He was disgusted,” pitching coach Mel Stottlemyre said of Brown. “I thought he was pretty good. We got him up to 78 pitches. We’re right on target. We’re moving up in the right direction.”

Robinson Cano hit the Yankees’ only homer, a two-run, pinch-hit shot off reliever Joe Roa.