This is going to be a very important summer for the Boston Celtics . Their two stars are about to wrap up the middle third of their careers, so while there is still time to get another championship with this core, that time is going to run out quickly. They have to hope the lessons they’ve learned this past season can help them build a proper supporting cast around Jaylen Brown and a fully healthy Jayson Tatum at the start of next season.  Here are the five biggest questions they are facing this offseason: How much help do Tatum and Brown really need?  This is a new question that popped up after the playoffs ended, and I think it informs the entire offseason.  A more accurate way to ask the question might be “are Tatum, Brown, and role players surrounding them enough, or does Boston need a third great player?” Maybe not someone at their level, but someone kind of close who can make this a Big Two-And-A-Half?…