If a doctor one day discovered how to wear your heart on your sleeve, Jim Harbaugh would volunteer. Until then, he has to explain what’s going on inside.

Such is the case right now. Harbaugh, dubbed “Captain Comeback” two seasons ago — when he was considered a top-five quarterback in the NFL — now finds himself coming to the Silverdome on Sunday as a backup on the losingest team in the league.

Even worse, his throwing hand is just now healing from a broken bone. It’s not the injury he needs to explain. Harbaugh, a tough player, has been injured all his life. You break his nose, he breathes through his mouth. You break his heel, he walks on his toes.

No, it’s how Harbaugh got this injury that he needs to explain. It is not a wonderful story, and he coughs through a dry throat when he tells me.

“Jim Kelly made some comments about me on a TV show in Buffalo,” Harbaugh says. “He basically said I was a baby, that I fake injuries. He said if he were still playing, he’d tell Buffalo players to hit me in the mouth and I’d get rattled.

“Well, he was doing a game in San Diego, and I wanted to ask him where he was coming from with those comments. We went into a room and started talking about it. He said, ‘I call it the way I see it.’ One thing led to another . . .”

And you hit him, I say.

“I hit him,” he says. “I threw a couple of punches. Sometimes you have to stand up for what you believe in.

“I broke the bone while hitting him. I’ve heard he’s telling people I never hit him, but I don’t know why he would say that. I would assume he knew what happened, since we were both there.

“On the way to the elevator, I felt my hand swelling up immediately.”

The injury has taken Harbaugh out of the loop for the last few weeks. It has relegated him to backup status now. It may forever shadow his career.

If you had it to do over again, I say, would you?

“I’m a guy that trusts my emotions,” he says.

That’s a yes, I say?

“That’s a yes,” he says.

Always a Michigan man

Now, I have known Jim Harbaugh, 33, since he was a college kid throwing passes for Michigan. I have known him since he used to say “I’m so jacked!” and all but lift into the air with excitement. From those years forward, whenever I see Harbaugh, there is an intense yet faraway gleam in his eye, and his mouth will curl inexplicably into a smile. You might be tempted to call it a lack of attention. That’s inaccurate. He’s paying attention. He may not be paying attention to you.

Harbaugh has an inner voice. You know. The kind of voice that says, “You can jump off that cliff. It’s not so high . . .”

The voice was there when, as a child, Harbaugh ran onto the field at Michigan, despite repeated warnings by his father, an assistant under Bo Schemechler, to stay away.

“GET THAT KID OFF THE FIELD!” Bo screamed.

It was inside Harbaugh years later, when, as quarterback for Bo, he predicted Michigan would beat Ohio State the week of the game. (He delivered, much to his and Bo’s relief.)

It was inside Harbaugh when, as a rookie with the Chicago Bears, I asked him how it felt to be a backup to Jim McMahon, then arguably the most famous quarterback in football.

“Things are gonna change,” Harbaugh said. “He won’t be the starter for long.”

It was there when he took Indianapolis to the AFC championship game in 1995, and threw a Hail Mary pass on the game’s final play that fell inches from victory and a Super Bowl.

Until that point, Harbaugh’s brio always had been celebrated. He got a huge new contract. He was profiled in every sports publication.

But then things changed. The Colts didn’t reach their 1995 heights in 1996. And this year, they lost their first 10 games.

And then Jim Kelly opened his mouth . . .

The old college spirit

I have known many pro athletes who see their college years as nostalgic. Given Harbaugh’s current woes, I ask whether he misses the old college spirit, especially now, during Michigan-Ohio State week.

“I don’t miss that spirit,” he says. “I have it in the pros.”

And despite all that has happened, it seems that he does. He sounds confident he will get his starting job back eventually. He sounds confident the Colts will turn the corner. He has a bet with a teammate, an Ohio State grad, on Saturday’s game, and the loser has to wear the other guy’s school outfit of choice for a week.

“I’m looking to get a Michigan band uniform,” Harbaugh says, laughing. “I want him walking around in the gold pants, the whole thing.”

I know Harbaugh is not where he wants to be. He doesn’t like losing. He doesn’t like backing up Paul Justin. But I don’t agree at all with Jim Kelly’s assessment. Few men have taken more physical punishment than Harbaugh, who has worked behind some soft offensive lines. He doesn’t get rattled. And he doesn’t need to fake injuries; he’s had enough real ones.

This doesn’t condone what Harbaugh did. But I wasn’t there when he threw those punches, and I don’t know where they landed.

I do know where they came from. They came from the heart. Sometimes, it takes you places you don’t want to be. Sometimes you go anyhow.

Mitch Albom will sign copies of his new book, “Tuesdays With Morrie,” 7-8 tonight at B. Dalton in Livonia Mall. To call in personal inscriptions for later pickup, contact Little Professor in Plymouth at 313-455-5220 or Barnes & Noble in Rochester at 248-853-9855. To leave a message for Albom at the Free Press, call 1-313-223-4581.