Despite spending 2018 in club rugby exile after being axed by Reds coach Brad Thorn, the 30-year-old playmaker showed why he will be in the mix for this year's World Cup.
The 70-test Wallaby lit up GIO Stadium with great mate Will Genia on Friday night as the Rebels started their season with a 34-27 victory over the Brumbies.
Former Wallaby Greg Martin called Cooper's performance the most commanding from an Australian fly-half in three years.
Melbourne coach Dave Wessels is delighted the Cooper is wearing the Rebels' No 10 jersey.
"(Cooper's) probably a once-in-a-generation player, he can do some things that make you go 'gee, how did that happen," Wessels said.
"He played to a framework that he's helped create, that he believes in, and I think he enjoyed himself out there."
Rebels captain Dane Haylett-Petty said Cooper's composure on and off the field was already benefiting the team.
"We know how much flair he can bring, but him and 'Willie' (Genia) led us around and even we were down, we felt calm," Haylett-Petty said.
Brumbies captain Christian Lealiifano said it was good for Australian rugby to have Cooper back but thought the halves duel was evenly matched.
"I felt like they (Genia and Cooper) didn't contribute as much as I thought they would, I felt we were in control, but the times we weren't they punished us," Lealiifano said.