The Boston Celtics are headed to Philadelphia for Game 6 of their opening-round series . The Celtics are still heavy favorites for the game and series, but there are some bits of panic wafting through the Celtics-sphere.  Here are six thoughts about the Celtics heading into Game 6.  ► The Celtics will be fine.  They played their worst quarter of the season in Game 5 and they were still only down six with 5:58 to go. They had some wide open, very clean, usually automatic looks throughout the next few minutes. The Sixers aren’t winning three in a row against these Celtics.  At the same time, the Celtics are doomed in the playoffs if they don’t fix the fixable stuff. Offensive execution has to be better. They need to be able to set their defense, get stops, and get some runs together against the Sixers.  This is something we heavily discussed on the new Locked On Celtics podcast:  ► I have no idea why Jaylen Brown was so intent on attacking Paul George in Game 5.…