Boston Celtics secure depth with contract extensions for Robert Williams III, Josh Richardson

BOSTON -- The Celtics have signed center Robert Williams III and guard Josh Richardson to contract extensions, the latest in a flurry of moves by Boston as it tries to shore up its roster before training camp.

"Rob has worked hard to turn himself into a very impactful player, and his talent and selfless play give him an even higher upside to reach going forward," Celtics President of Basketball Operations Brad Stevens said in a statement Tuesday. "Rob has embraced being a Celtic from Day 1. He is a great teammate and is completely committed to getting better. We're excited that he will continue to do so here in Boston."

A source told ESPN that Williams' deal is for $54 million over four…