That's too harsh, IMO.
First of all, Bonner never has had a good playoff series -- whether he was "needed" or not -- so just based on that, Green has surpassed Bonner. Bonner has a long, painful history of choking that even can be traced to how he tails off at the end of the regular season. Green had one horrific series.
(And we shouldn't short-change how good he was in the first two series. He was averaging 10+ points in less than 25 MPG on 50% shooting from the floor and >45% shooting on three-pointers. His D was also much better than usual. Advanced stats, from plus/minus to APM to RAPM had Green as one of the best players on the whole team -- with many of those stats pointing to him as THE best player on the team. Yeah, easy compe ion but Green delivered to the maximum of his ability in the first two playoff series of his career. That's pretty darn good and should shed him of the Bonner label in itself.)
Why did Green suck* against the Thunder? I see three possibilities:
1. He choked. The pressure got to him and he couldn't withstand it.
2. He was due for some regression to the mean since he had been shooting damn near 50% on threes for the previous couple months.
3. He got unlucky and the randomness of the sport of basketball came up snake-eyes at the wrong time.
The only one of those three to be worried about is No. 1. And even then, we should only be partially worried because plenty of young players struggle early in their playoff career before adjusting to the pressure and learning to thrive. So, yeah, if we want to view Green in the worst possible light, then we can hold that series over him until he proves otherwise. IMO, what is more logical is to keep an open mind. Yes, it sucks he failed against the Thunder but there are a lot more explanations other than to say he's undoubtedly a choker.
As others have mentioned, plenty of Spurs have done poorly in their first time in the playoffs for the Spurs only to bounce back. Tony Parker destroyed Gary Payton as a rookie but he didn't carry that level of play throughout those playoffs. He, as CoM is quick to mention, had his ups and downs in 2003 ... yet he's definitely not in the Bonner category. Ginobili was up and down as a rookie in the playoffs (he shot 20-something percent in the Finals and wasn't that great against the Mavs or Suns) ... and he turned out to be the ultimate winner. , Tim Duncan wasn't that good in the Portland series in 1999.
Other examples: Stephen Jackson sucked in 2003 against the Lakers (if the Spurs lose that series, Jack would have been remembered as a choker rather than the clutch guy you remember). Bruce Bowen was terrible in his first playoff series with the Spurs (vs. the Sonics). Horry sucked in 2004. Claxton sucked for most of the 2003 playoffs. Kerr sucked every series until that Mavs series during his second stint with the Spurs.
You can go right down the line and just about every player on the Spurs has had a horrible series. Sometimes they were saved by their teammates so those series are forgotten. Sometimes they weren't saved and they are remembered as chokers. If the Spurs beat OKC and Green bouncing back against the Heat, he gets >$6M per year in free agency and is hailed as a hero ... pretty much just like Jack in 2003. It's fickle.
We need to be careful handing out Bonneritis claims. That's a serious diagnosis and one that should indicate that beyond all shadow of a doubt, a player is a choker for life and can only be trusted in the regular season. Maybe Green fits that bill but it's way too early to claim it as a certainty. One bad series does not a Bonner make.
*despite Green's sucking, the Spurs still outscored the Thunder when he was on the court during that series.