Jim Fassel -- the former NFL head coach who led the NY Giants to Super Bowl XXXV -- has died ... and his ex-star player, Michael Strahan, is heartbroken.
Fassel passed away Monday in Las Vegas after suffering a heart attack following chest pains, his son, John, told the LA Times.
And, when Strahan was asked about his former coach on "Good Morning America" on Tuesday ... the ex-pass rusher explained he was crushed to learn of the passing.