Referee Mark Smith yelled and immediately called a timeout ... while Aljo lay on the Octagon floor, clutching his head.
Smith called in doctor ... who ultimately determined Aljamain -- who was clearly wobbled -- could NOT continue.
Because the knee was deemed intentional (Smith warned Yan right before he threw the strike that Sterling was a downed fighter, and therefore illegal to knee him) ... Petr was disqualified.
After the fight was called, Sterling laid on the mat, crying.
Aljo just addressed the stoppage on social media, writing ... "Not the way I ever want to win a fight. It was close, competitive and filled with action. I felt the knee was intentional, especially after the ref announced I was a downed opponent, so I didn’t expect to be hit."
Sterling continued ... "Yan is a bad dude! WE WILL DO IT AGAIN!"
Yan also just addressed the DQ on Twitter, saying ... "I apologize and wish speedy recovery toI didn’t mean to throw an illegal strike, I just made a big mistake and paid for it."
Prior to the illegal strike, the fight was close ... with Yan leading 2 rounds to 1 on 2 of the 3 judges scorecards.
It's the 1st time in UFC history that a title has exchanged hands 'cause of a disqualification.