Patrick Mahomes is devastated after tearing his ACL in Sunday's game ... writing an emotional message on social media -- questioning "why this had to happen" to him.
The Kansas City Chiefs' superstar suffered the injury with less than two minutes left in a Week 15 matchup against the Los Angeles Chargers at Arrowhead Stadium.
The Chiefs confirmed it was a torn ACL in his left knee ... an injury the 30-year-old is now trying to come to terms with for the first time in his career.