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 wins and one playoff series win was shoddy enough that people began to wonder if Brad Stevens—perhaps the league’s most admired young coach—was really all that. 

Then, by every available measure, the team deteriorated over the offseason. They lost Kyrie to the Nets and Horford to the 76ers. Aron Baynes became a Sun, too, which now suddenly seems like a move that matters a ton. The Celtics replaced them with Kemba Walker, Enes Kanter, and role players. And so a middling team appeared to get worse. It’s hard to find a preview suggesting the Celtics were serious threats to win the East.

Yet it’s one month into the season, and they have the N…