PRINCETON — Hulking centerback Oguchi Onyewu may well be the biggest X-factor in the U.S. National Team’s World Cup hopes, both figuratively and literally. The knee surgery he had seven months ago tomorrow – and whether he can return from it – will play a huge role in how far the U.S. can go in South Africa.

When the 6-4, 210-lb. defender suffered a torn patella tendon in the last qualifier on Oct. 14, many questioned whether he’d be able to come back in time. Watching him run with a seeming limp in today’s practice didn’t quell those questions, just underscore his importance to the U.S. hopes in South Africa.

“From Day 1, people were like “Oh, Onyewu’s going to miss the World Cup.’ And I’d read it and then turn the page to the funny section,” said Onyewu, 28. “I always say that I know who I am and I know what I’m capable of, and nobody from the outside can tell me what I’m capable of doing besides me.

“You’re telling me (about a limp); I don’t feel it. I feel fine…I’m not concerned. I’ve been doing double-days in Milan so the rust factor won’t be an issue.’’

Coach Bob Bradley didn’t address the media yesterday, but Onyewu’s teammates were both ecstatic at his presence and acutely aware of his importance.

“It’s just good to have Gooch around. He seems like he’s fit. He seems fine. I asked him, all his movements are good. He has the normal stiffness you have from a knee injury, but he’s functioning great,’’ said Everton keeper Tim Howard, a North Brunswick (NJ) native and former MetroStar.

“Gooch is one of those guys who is a big presence, even among us older players. He came into the hotel (the other) night and it was like Big Papa’s come back around and everybody was happy. It’s just great to have Gooch around.’’

The 28-year-old has been a mainstay in central defense, usually alongside Carlos Bocanegra, both in the disappointing 2006 World Cup and in the equally-impressive Confederations Cup run last summer in South Africa.

“He’s a good, strong presence in the center of the back,’’ said midfielder Michael Bradley, Bob’s son. “He’s been a guy who’s played in just about all the big games for the national team for the past six or seven years. He’s a leader a strong presence back there and certainly someone we rely on.’’

He showed that leadership when – after being limited to just one game for AC Milan _ he signed a one-year extension this week with the Serie A club and offered to play that final season for free.

“It was just a sign of good faith,’’ Onyewu said. “The club was able to stick by me. (I wanted to) show my dedication back to them.”

Few who saw Onyewu’s injury could question his dedication, an injury severe enough he said “When I saw my knee, that’s when I knew something was wrong. My kneecap was in my quad muscle.”

And that’s when the pain just began.

Onyweu said doctors ripped tendons from the bottom of his knee and sowed a “wire-like material’’ through them. They drilled holes through the kneecap and threaded them through to strengthen it, giving one knee a different appearance. But Onyweu said “it could look like this (Gatorade bottle) as long as I can play.’’

Then came the rehab, done in Italy, L.A. and Delaware, the latter alongside Charlie Davies, whose comeback bid hasn’t had as happy an ending.

“You reevaluate things, look at yourself in the mirror (for) that internal motivation,’’ said Onyewu. “Having Charlie there made it easier and we motivated each other. There was a lot of soul searching. It’s not easy to watch your team on TV and know you’re not capable of doing that yet.’’

Tuesday he’ll get his chance, when the U.S. plays the Czech Republic in a tuneup in Hartford. If he can impress there, and perhaps four days later in Philadelphia, he can reach another World Cup. The last wasn’t kind, called for a penalty he termed “an injustice’’ in a 2006 loss to Ghana.

“A lot of people are saying I won’t come back like I was,’’ Onyewu said. “I’m going to agree. I won’t come back as I was: I’m going to come back stronger.”