I know that Jeff Green has put up 2 season-high scoring numbers when he's started, but there are a few reasons why I think he should remain on the bench.
1) We need bench scoring.
The Celtics have always wanted a strong bench right? Well now the Celtics have one, but the fans are clamoring for our top bench scorer to leave the bench and join the starters. That puts Brandon Bass on the bench, and for Jason Terry to lead the bench unit. Jason Terry has played well for the last 2 months, but he's not being given the minutes to be the 15-20 point scorer he was meant to be. Green has shown to be the 15-20 point scorer, and he's easily the most consistent scorer off the bench.
2) Jeff Green will not be as effective as a starter.
Jeff Green has only started when Pierce and Kevin Garnett have sat out. In the four games that Green has started he's averaged close to 14 shot attempts. When he has more attempts he's scored more. With that being said, Green's attempts will go down because he has to share shots with Pierce, Garnett, and Bradley. Less attempts, less points, more people clamoring for Green to get back on the bench.
Besides, Paul Pierce has had a fantastic month of March averaging 18.8 points, 7 rebounds, and 6 assists, shooting 52.2% from the field, and 48.5% from 3-point range in 32.6 minutes. He's also averaging a healthy 11.5 field goal attempts during March. The month of March hasn't been friendly to Garnett, but he's been averaging close to 13 shots a game. Garnett is going through a slump, but he's a player that deserves and is accustomed to taking 10-15 shots a game.
3) Brandon Bass.
Brandon Bass has played wonderfully since the beginning of March. I've even made a quick fanpost about what he's been able to do thus far. I understand that Green is outplaying him, but that doesn't mean that Bass is playing poorly. Brandon Bass hasn't done anything that warrants him getting benched again in my opinion.
The only way I could put Green in the starting lineup for the rest of the year is if I put Pierce on the bench for the rest of the year, but as I've said, there's no reason to. Pierce is playing his best basketball of the season. You don't have to have all of your best scorers starting. Westbrook-Durant-Harden didn't share a starting lineup for a whole season. The same could be said for Marion-Nowitzki-Terry, or Duncan-Parker-Ginobli.
Green is the most consistent bench scorer on the Celtics, Jason Terry has had good percentages, but his field goal attempts vary a lot, while Green's has been relatively consistent. Green can play 40+ minutes easier than Pierce can, but that doesn't necessarily mean you should get him accustomed to it. Green is just fine subbing in for Pierce, and in crunch time they can share a court together alongside Garnett.