The NBA is not a simple game of the sum of each individual part equals the whole. Unfortunately, one player can be special, but his addition to the team is negligible depending on the makeup of the team (Irving). It is also true that players that are doing too much can be replaced by other players more effectively (Horford). It is also also true that players that are not given the opportunity to perform well can be overshadowed by more experienced players (Morris), even if they are better.
It is also also also true that basketball is a team game that requires players playing together at a high level of synergy.
I think Walker is not as talented as a player Irving, but I think he will be a better player for the Celtics.
I think Horford is a good player but was asked to do too much last year, and that Kanter, Williams, the Frenchman, and Theis will be better fits.
I think that Tatum and Brown stepping out of the shadows on offense will lead to a better offensive team.
And I think that the Celtics as a whole will learn to play together better than was possible last year.