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Looking for feedback about the premise.

Couple of questions.

Are there three different species right off the bat? Humans, Johnny Stello's species and another alien species? Or is it just the two?

What is going to drive the story? Character or plot?

It reminds me of some of the earlier Harry Harrison stuff, where there is a conflict, the humans are prevailing but just barely and the continued survival is well.. questionable.

I enjoy that kind of book so if the writing supports the plot I would read it.

Boxing champion Sam Boulder is tricked into leaving Earth.
After reading the premise and coming back to this I think so? While the rest may provide a framework for Sam to begin his story, what is the story you want to tell? Why will we care about him? What does he have to lose? The rest of the premise didn't seem to elaborate on him.

Humans are on the verge of self destruction.
Why? What from? Some future epidemic, or perhaps a contemporary pessimism of nuclear proliferation? When is this set, now, in the near future, or 3000 years in the future?

On sports promoter Johnny Stello's planet, he encounters
Who encounters? Sam or Johnny? And if this is a contemporary setting, is Johnny Stello an alien? The introduction sports promoter Johnny Stello gave me the first impression that it's someone from NBC, then I hit planet and got stuck.

Nor do they want to risk imitating negative aspects of human behavior. So they recruit humans and use technology to protect them in the arena
Why would the gladiator harvest dry up? Couldn't they breed humans? It doesn't sound like they're killing them off in their combat sport.

the leadership worries about the impact
This comes back to that Sam thing - the rest of the premise has more to do with the aliens, but if Sam is our eyes and the character we care about, do we care about their culture? Will he wrangle their worry into help for Earth? Perhaps if told through his eyes and motivations, I might be more interested in what some distant alien leadership is worrying about.

I think that posting a small synopsis like this is a great idea, and probably also great practice for honing a story or backcover blurb or query letter. And I am looking forward to more info down the track