BY DAVID THORPE

A season ago, the Celtics were the NBA’s great mystery. Their young players like Jayson Tatum, Jaylen Brown, Terry Rozier, and Marcus Smart were the envy of the league. They also had a superstar in Kyrie Irving, and arguably, another in Gordon Hayward, to go with the veteran presence of Al Horford and Marcus Morris Sr. But ultimately, 49 …