Alvarez (54-1-2, 36 KOs) dominated his only bout this year, repeatedly hitting Smith (27-1, 19 KOs) with straight rights and right hooks to the head and punishing his upper arms over 12 rounds.
Matchroom promoter Eddie Hearn said doctors believe Smith tore his left bicep or triceps muscle. There was a noticeable protrusion coming from the bottom of Smith's arm beginning in the fourth round from Alvarez's repeated blows.
Alvarez previously knocked out Callum's older brother, Liam, on September 17, 2106, at AT&T Stadium in Arlington, Texas. Many speculated Callum wanted revenge for the ninth-round knockout, but Alvarez dominated the younger Smith, too.