"I was terribly nervous, more than I am when I pull on an England shirt. I was in awe of it all.
"It was a pleasure for me to be out here with Ricky and to carry the belt in the ring was a great honour. It was a big experience. I was overjoyed. It was brilliant — no, absolutely fantastic.
"It is my first time in Las Vegas. And what a really great place it is."
Rooney spent most of the fight out of his front-row seat as he yelled encouragement to his pal — who supports United's arch rivals Manchester City.
"In the dressing room before, I just stayed out of the way as I know when I play I want to get focused. But during the fight I was jumping up and down."
Teammates Rio Ferdinand, Wes Brown and John O'Shea also swelled the ringside crowds — as did Chelsea's Joe Cole and Shaun Wright-Phillips plus Shaun's brother Bradley, a Southampton star.