Fassel expressed his opinion Tuesday while co-hosting The Coaches Show podcast on NFL.com.
"He'll go down as the greatest quarterback of all time," Fassel said. "The numbers he has already put up as a young quarterback, far superior to anybody else that came in the league in their first four, five or six years the numbers he's put up. If he keeps at that pace ... Whoa!"
Fassel coached the Giants for seven seasons, leading them to two division titles and a Super Bowl appearance before being fired in 2003 after a 4-12 season.
Luck has taken his team to the playoffs in each of his three NFL seasons. In 2014, he set career-highs in completion percentage (62 percent), passing yards (4,761), touchdown passes (40) and passer rating (96.5).
The Colts take on the Denver Broncos on Sunday in the AFC Divisional Playoff.
- Scooby Axson