Health is a key factor for all teams but not necessarily an equal factor. Teams that come to mind, LAL, MIL, PHI, PHX, LAC, DEN all have injury worries that are probably at least as concerning as Boston's. GSW, CLE, NYK are some potentially top teams that don't seem to have as much injury risk going into the season, but that can change. People like to couch predictions by saying things like "if healthy" this team or that team is better. But injury risk is a factor that you have to consider when you make a prediction or otherwise assess a team.
I look at the Celtics this way. I think Horford is going to plug along the next couple of season much like he has the last couple. His minutes will be managed but he most likely will be there when they need him to be. I also think they are going to figure out a way to keep RWill on the court enough. The wild card is Porzingis. We have him 3 seasons. Probably at least 1 of those seasons will be impacted by injury in some significant way. You just have to expect that. But I think if we have Porzingis and at least one of Horford or Williams at any given time, that is enough to allow us to contend for a title, to be competitive against any team in a playoff series.
The other player whose durability seems to be a concern is Brogdon. I think he is going to be fine. He is the 6th man, a perfect role for him. Plenty of opportunity to work around things and manage his minutes. If he was needed to be a 36 min starter, I might feel differently.
In the end, some teams are going to be healthier than others going into the playoffs. This is a factor for every team, not just the Celtics. Something could happen to Tatum or Brown or White, who knows. But based on the durability history of the individual Celtic's players and diversity of good players that we have, I think we are going to be a solid title contender at least 2 of the next 3 seasons, with 1 season derailed due to untimely injuries. And to take it one step further, I like our chances going into this season.