Some thoughts...
#1 - A lot of people like to say starting doesn't matter, but it sort of matters, doesn't it? You get to set the tone early, you get a rhythm going straight out of warmups. You aren't coming in cold and trying to single-handedly change the pace of the game. I suspect it's harder to "let the game come to you" when your role is to come in cold and "BE AGGRESSIVE!" with limited offensive opportunities. There are still games where Green plays less than 30 minutes and people act like he's "inconsistent", because he doesn't score double digits in those games. A lot of that comes down to role. He's probably your 2nd best offensive player right now... if you bring him off the bench and give him 6-8 shots opportunities, do you really expect him to have the same kind of performance if you start him, play him 38 minutes and give him 15-20 shot opportunities? It's silly.
#2 - Green's been sensational. In the 4 games he's started, he's averaged 24.5 points on 60% shooting... but even in his limited "bench" role, he averaged 15 points last month (51%/41%/86%) and this month he's averaging 17 points (48%/42%/78%) all while averaging about 31 minutes a night. He's doing everything you want him to do and more... and yet people still act like he's "maddeningly inconsistent". I don't get that at all. Even James Harden had games where he couldn't put up double digits off the bench... you start him, make him a top option and give him consistent minutes... and "shockingly" he's averaging 10 more points a night. Not saying Jeff would average 27 a night, but it's 19-20 is totally reasonable with him.
#3 - If you're concerned about starting Green at PF against bigger teams... I'd be in favor of just moving Pierce to SG in those games and starting Jeff at SF... match size with size. I still wouldn't make my 3rd best player (and 2nd best offensive player) come off the bench.
#4 - We have a capable "microwave" 6th man already... his name is Jason Terry. And it's not like we have a separate 2nd unit from our starters... people act like starting Jeff Green would destroy our bench. That's nonsense. There's 48 minutes in a game. If you start Pierce and Green together, it doesn't mean you have to rest them at the same time. Come playoff time especially, Pierce is known to get about 39 minutes a night. If Pierce and Green are ideally only getting about 10 minutes rest a night each... who says they need to take those 10 minutes at the same time.