By a giant asian-american from Korea. Here's the story from the AP.
It took Hong Mon Choi, no relation to KJ Choi I suspect, a mere 1 minute and 17 seconds into the match. Almost as much time as it took Jose to start fingering roided up ball players after he squandered his fortune.
"The referee stopped the fight when the 7-foot-2, 330-pound Choi knocked Canseco to the mat and started punching his head." Yikes.
Canseco appears to injure his knee rounding second, just before the giant Korean goes Nolan Ryan on Canseco's coconut and then pounces on the former bash brother. The slowmo makes it look like this was staged, in other words, it had all the drama of a Los Angeles Laker game 7.
If you saw Vai Sikahema knock Canseco out just after signaling for a fair catch in another Canseco prize fighting classic, you had to know this was coming. Choi is apparently a legit MMA fighter.