INDIANAPOLIS — It took Chuck Pagano less than nine months to instill his fighter’s mentality and hopeful spirit in the Indianapolis Colts.

He will need both to survive the biggest battle of his life — leukemia.

In a somber press conference Monday, the Colts announced that their new coach had been hospitalized for cancer treatment and probably would not return to full coaching duties this season. He will be replaced on an interim basis by offensive coordinator Bruce Arians.

“He will do fine,” Arians said, his voice cracking as he recalled his own fight with prostate cancer in 2007. “I know him. He’s a fighter. He’s survived tough times already in his life. As a cancer survivor myself, I know that these first few days are really hard on you but as he and I talked yesterday, it’s just a matter of time.”

The news hit hard in all corners of the team complex.

Team owner Jim Irsay, who began his career as a Colts ball boy in the early 1970s, said the only comparison he could come up with was Vince Lombardi’s cancer diagnosis during the summer of 1970.

New General Manager Ryan Grigson read stoically from his prepared notes, and Arians struggled to hold back tears.

After practice, players signed a card that read in part, “We are in your corner 100 percent. Get rest, but we can’t wait to get our leader back.” The usually jovial comments were replaced by concerned looks and serious discussion about life — not football.

“When I first heard about it, my heart dropped,” cornerback Jerraud Powers said. “You think about your family members or someone that’s actually been affected by it. But Chuck will fight this thing and he will beat this thing, there’s no doubt in my mind.’

It didn’t take long for the Colts to figure out how to honor the first-time head coach who rekindled excitement in the locker room and around town after the Colts’ awful 2-14 season a year ago.

“I asked Mr. Irsay if we would leave the light on in his office permanently till he comes back and we are going to do that,” Arians said.

The news trickled out publicly just as players and assistant coaches were returning to the team complex after the Colts’ bye week and one day before Pagano’s 52nd birthday.

Indy (1-2) hosts the Packers (2-2) on Sunday.

Pagano was admitted to an Indianapolis hospital last Wednesday to begin treatments for acute promyelocytic leukemia, an illness in which the bone marrow produces abnormal white blood cells that interfere with healthy blood cells. Symptoms can include weakness, weight loss and easy bruising or bleeding.

Redskins quarterback Robert Griffin III wrote on Twitter: “Football is football. But Life is more important. Sending Prayers up for Coach Chuck Pagano during this trying time.”