Austin hadn't heard of I.T.'s injury holding up the deal ... but told TMZ Sports NBA fans can chill out about it cause the Cavs and Celtics are handlin' business.
"They're just doing their job -- doing their due diligence. I'm sure the deal will go through." Rivers said.
Basically, they'll find a way to make it work -- cause it's a win-win for both squads.
Word is the Cavs (so, LeBron) want even more now, though ... and Kyrie is somewhere crying (kidding).