Four days a week, Mariano Rivera makes the trip from his Westchester home to Manhattan. Four days a week, he puts in three hours of stretching, exercising and sweating designed to get him back from his torn ACL and on a major league mound.

But when?

Yankees general manager Brian Cashman said yesterday “in terms of 2012, he’s out.”

Rivera, though, has always been good at defying expectations. In the most informal of surveys, I went around the visiting clubhouse at Fenway Park over the weekend and could not find a single person in a Yankees uniform who did not think that at some point out of nowhere soon we would hear, “Mo has begun running” or “Mo is down in Tampa doing some long tossing.” Such is the mystique and respect he engenders.

The clock is working against him returning this season. But so much is working for him and that comes from the person who is most involved in his rehab; in those four-day-a-week, three-hour sweat sessions.

Dr. Keith Pyne made it clear on several occasions in a phone conversation with The Post that the ultimate authorities on when Rivera will return are his surgeon, Dr. David Altchek, the Yankees and Rivera himself. And he said the working theory is, “Right now, he’s shut down until next year.”

But the key words in that reply were “right now.” For in our conversation yesterday, Pyne also said Rivera is “working his butt off,” is well ahead of schedule in rehab and “is itching to get back.”

GRADE THE YANKEES AT THE BREAK

He added, that Rivera “will be in the best position to accomplish that goal [pitching this year]. He’s got everything it takes to accomplish that. … If I was putting money on it, I would put my money on Mo.”

Rivera, perhaps the most durable closer ever, tore the ACL in his right knee on May 3 in Kansas City while shagging balls in batting practice. Due to a blood clot, his surgery was delayed to June 12. But in listing reasons to believe in Rivera’s speedy recovery, Pyne cited the delay in surgery because it enabled Rivera to do extensive pre-hab with Pyne and rehab specialist Ben Velazquez.

“This is a very detailed guy and he did everything right [pre-surgery],” Pyne said. “He strengthened, he got range of motion. He was very functional before surgery. I don’t want to put a percentage on it, but he reduced recovery time by a lot.”

There were other factors Pyne believes are accelerating recovery. It turned out when Altchek went in that Rivera had a partial, not a full tear. Pyne believes the recovery time would need to be longer if it were Rivera’s landing left leg because of the impact and twist necessary at the end of a motion.

Mainly, though, Pyne attributed the rapidity of recovery to Rivera’s physical and mental makeup. Pyne had not known Rivera before this rehab, but he has learned what those who have followed Rivera’s career are intimate with: The right-hander has freakish flexibility, athleticism and determination.

Pyne has rehabbed more than 1,100 athletes in his 22 years and needed to keep his eye on the clock during our conversation because he had to go work with Travis Beckum, the Giants tight end who tore an ACL during the Super Bowl.

“[Rivera] is special, in the top 10 percent of athletes I have worked with,” Pyne said. “One great advantage he has is his lifestyle is very conducive to longevity. He eats well, prepares well. He is very cognizant of how to treat his body. That has put years onto his career. He is a special athlete with a special nervous system. He has great flexibility and joint mechanics. … But the most special thing about him is his cranium. He works and is focused on the task at hand. He attacks this like a professional.”

Pyne believes Rivera has progressed so far, so fast that the closer could do more intensive load-bearing maneuvers such as running and throwing right now. But the protocol calls for at least 45 days after surgery before advancing in that way, which would be late July, at the earliest.

“You have to let the ACL scar down, it is just being cautious,” Pyne said. “So right now we are establishing all the neuro-connections in his knee, foot and back going to the brain so that when he starts to load there are no complications. It is Dr. Altchek’s call he will tell us when [Rivera] can begin weight bearing. In the meantime we are getting him ready to accept load. Once you do that, that is the hard part and he is already ready to go. When you do that, when you get to load bearing, recovery time speeds up immensely.”

But how fast? Brewers pitcher Yovani Gallardo tore his right ACL on May 1, 2008, returned Sept. 25 and started Game 1 of the Division Series. But he was 22. Rivera is 20 years older.

“For me there is biological age and chronological age,” Pyne said. “I don’t see Mariano Rivera’s biological age as 42. He is mechanically and physically not a 42-year-old. He has the genetics of a much, much younger guy. This is not his last season. I cannot make the determination [of the exact date he will return]. But physically he will be able to do whatever it takes to pitch again.”