The 31-year-old suffered a knockdown, but also downed Spellman on his way to a victory considered by many as an upset.
He conducted post-fight interviews but then collapsed in the dressing room afterwards and was taken to Sheffield's Royal Hallamshire Hospital.
Boxing stars such as Frank Bruno, Ricky Hatton and Curtis Woodhouse have all paid tribute on social media site Twitter.
Bruno said he was "Very saddened to hear a boxer has lost his life," while Hatton said: "Thoughts to his friends and family. And to anyone involved in the show."
Westgarth, who was trained in Sheffield by Glyn Rhodes and lives in Penistone but is originally from the North East, was knocked down on the ropes just before the bell of round 10.