We actually would look good on the Boards with this five. Against big team's it could work.
Then sub Baynes and Brown and Maybe Horford for Smart, Tatum and Morris
I'm going to get up on my hobby horse again about rebounding:
Offensive and defensive rebounding are different and should be treated separately. Short version: how much commitment a team makes to offensive rebounding is variable. Defensive rebounding commitment is not variable. Defensive rebounding is a lot more important than offensive.
Brad Stevens' offense lifts the bigs to clear the lane for drives. The commitment to offensive rebounding is not great, and that's not likely to change much next season.
Defensive rebounding is not optional. Yes, you can get away with less if you force more turnovers; last year Boston was middle of the pack for that, and it's not obvious that the Celtics would be any better at it this year.
Does Baynes start, and would it make a difference in defensive rebounding, compared to last season? It should make a difference at one position - he was substantially better than Amir last season. I can see arguments pro and con for him starting, and his defensive rebounding might be the best pro argument.
Here's the defensive rebounding comparison with your starting lineup and last year's:
Irving 7.5% / Thomas 7.0%
Brown 14.4% / Bradley 16.2%
Hayward 15.4% / Crowder 17.3%
Baynes 21.6% / Johnson 17.0%
Comparing in this way, there's a slight advantage to 2018. If Morris starts, the numbers swing bigly the other way.
I think that this is going to be a problem.