He is a big upgrade over our present SF.
This is true. But once the C's have signed Hayward and they have him, Crowder, Brown, and Tatum/Jackson (not to mention Bradley and Smart) what's the plan from there? Bringing in Hayward will necessitate a follow up move unless the C's plan is to totally abandon the idea of playing big men.
Hopefully,
Plan A:
Find placeholder big
Draft 4/5 with next year's Brooklyn pick and/or Lakers pick
Blend Thomas, Bradley, Hayward, Horford, Draftee as your top 5, with other players (i.e. Smart, Brown, this year's pick) competing for playing time
Plan B:
Steal Porzingis from Knicks
Field Porzingis, Horford, Hayward, Bradley, Thomas as your starting 5 this season, give or take a trade piece