Indianapolis Colts' head coach Chuck Pagano has been diagnosed with a treatable form of leukemia, owner Jim Irsay announced.

"It's been a very difficult week," Irsay said. "...I am optimistic. I feel with every fiber of my body --- and I know Chuck feels the same way --- that he can beat this thing."

Pagano is unlikely to return to coaching in full this season.

Larry Cripe, Pagano's doctor, said the goal of the treatment is to cure the disease and have Pagano return to coaching. But, Cripe added, "The process is long and complicated." 

Offensive coordinator Bruce Arians will take over as Colts' head coach in Pagano's absence.