Is Irving good enough? I'm not even sure this is the right question.
Irving is not going to be playing in the Finals by himself. This is not Cleveland pre-Lebron. He's on a team with one of the best wings in the league (Hayward), one of the best do-it-all big men in the league (Horford), one of the best coaches in the league (Stevens), a good bench and a boatload of young talent.
Of course he's good enough to win a title here. He doesn't have to be Russell, Bird, or Havlichek to be good enough.
Another thing to keep in mind... He can be an adequate defender, which would be a big leap from what IT was giving us (and yes, I was a huge IT guy). If you're a C's fan and you're being honest with yourself, you'll admit that there were plenty of games when the team was getting killed defensively with IT on the floor, and they needed IT's 4th quarter heroics just to repair the damage. Irving isn't exactly Joe Dumars on defense, but I expect the Celtics to take a major step forward defensively this season, even without Bradley and Crowder.