Duffy's performance was all the more remarkable given that, in 2010, a freak collision in a training session caused an artery connected to his liver to be severed, prompting emergency surgery to save his life.
And Coleman, who has played alongside the 24-year-old for Everton as well as Ireland, was amazed by his display in the vital 1-0 win.
"Shane Duffy was incredible, in his first competitive game," he said. "Let's not forget he was nearly dead six years ago. He's such a good lad, we've been together since Everton, and I'm so proud of him."
Duffy himself admitted he did not even know he would be starting until he arrived at Stade Pierre-Mauroy, and he conceded it was difficult to settle at first.
"I found out when I got here but I always get myself ready," he said. "You never know what happens in football. I was a bit nervous for five or 10 minutes but just tried to settle down.
"It's hard to sum up as I didn't play a minute in qualifying, but I'm grateful for the boss picking me. I thrive off big games. That's the best night of my career for Ireland.
"They're probably the toughest team in world football to break down but we pressed them from the start and deserved the three points."
Robbie Brady secured the win which propelled Ireland into a last-16 tie with France, glancing home a header in the 85th minute following Wes Hoolahan's cross.
Coleman, who captained Ireland against the Italians, reserved special praise for Hoolahan's contribution, which came just minutes after he spurned a one-on-one chance to win the game.
"It's hard to describe the emotion when that goal went it," he said. "We didn't know if it was going to come, and it came, and it's a night I'll remember forever.
"You've got to give Wes a hell of a lot of credit. A lot of players would have had their head down after that, or sulked, but he got on with it and what a delivery.
"Robbie Brady's goal will be remembered for a long time. I'm so happy for all the players, staff, security, all the fans who have been incredible, well-behaved, singing all the way through. I won the toss and chose to shoot towards the Italians first because I knew the fans would suck the ball into the net."