Jimmy Butler is slated to make around $17.5, $18.5, and $19.5 million for the next 3 seasons.
Considering his talent and ability, that is a steal. That's nowhere near the current max contract that players of his caliber will be getting which will be in the $30+ million per year range. The Celtics do have a small window to compete considering they have Horford under contract for several more seasons and will have to pay IT and Bradley following next season.
They don't NEED to trade the 1st overall pick, but doing so isn't a bad idea either.