By the time Oleksandr Usyk and Anthony Joshua fought late Saturday night in London, Lenin Castillo joked with his team members as they watched that heavyweight title fight back at the event hotel.

BoxingScene.com has learned Castillo underwent a battery of tests at a London hospital and was discharged a few hours after his scary second-round knockout loss to Callum Smith on the Joshua-Usyk undercard. Smith’s damaging right hand knocked Castillo unconscious and left him convulsing violently on the canvas.

Medical personnel tended to Castillo for several minutes before he was removed from the ring on a stretcher at Tottenham Hotspur Stadium. Liverpool’s Smith expressed concern for his fallen opponent even after the official result finally was announced.

Despite the vicious nature of his knockout defeat, Castillo (21-4-1, 16 KOs) is expected to leave London for his home in the Dominican Republic on Sunday, as planned. Nevertheless, the British Boxing Board of Control likely will place Castillo on a lengthy medical suspension that should be recognized by commissions throughout the world.

Castillo, 33, lost by knockout for the first time in his 11-year pro career against Smith. His three previous losses to WBA light heavyweight champ Dmitry Bivol, former WBA interim light heavyweight champ Marcus Browne and Joseph Williams all went the distance.

The 31-year-old Smith (28-1, 20 KOs), a former WBA super middleweight champion, fought at the light heavyweight limit of 175 pounds for the first time in nearly nine years as a pro when he conquered Castillo. His spectacular knockout of Castillo also marked his first fight since Mexican icon Canelo Alvarez (56-1-2, 38 KOs) dominated the previously unbeaten Smith on his way to winning their 12-round, 168-pound championship match by unanimous decision December 19 at Alamodome in San Antonio.