Collen, who is awaiting the birth of his child with wife Helen, left for a family emergency, the article says.
Brown's first appearance was at Friday night's show at Hersheypark Stadium in Pennsylvania.
In a 2017 interview, Brown said: "Those guys are like family to me. Most people don't know this, but I go back with Def Leppard to about 1988 when they were touring behind 'Hysteria'. Phil Collen is a like a brother to me."
Brown and three others comprise Trixter, the Paramus-based glam metal/hard rock band.
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