The report, citing unnamed sources, said Pagano waited until the team's week off last week to get checked out after experiencing extreme fatigue and bruising since training camp. He has been in the intensive-care unit of an Indianapolis hospital since Wednesday, receiving treatment, the report said.
Early Monday, Colts owner Jim Irsay tweeted: "Colts n all NFL Fans..keep Coach Chuck n his family in your prayers..He is fighting with courageous energy n faith and WILL win this battle!"
Pagano, who is in his first season as coach, is expected to miss at least six weeks but plans to return.
According to the Indianapolis Star, he will be temporarily replaced by Colts offensive coordinator Bruce Arians.