“It’s just beautiful,” Willis said. “I love seeing him out there. I never saw it coming — I never really saw Tim as a coach. But since it happened, why not? When he’s working with other players, he’s the ultimate teacher.”

Before games, Duncan is back on display for fans who arrive early enough, warming up the reserve center Jakob Poeltl and the former All-Star LaMarcus Aldridge. Duncan has let his hair grow out extra long and has styled it in dreadlocks, and he typically duels with Poeltl and Aldridge while wearing a pair of camouflage Spurs practice shorts — along with an unmistakably bulky brace he still faithfully slides over his left knee.

In Dallas on Nov. 18 , Duncan, unlike any other assistant coach I’ve seen lately, could be spotted in the hallway adjacent to the visitors’ locker room doing situps on the concrete floor between preparatory sessions with Poeltl and Aldridge. Duncan also inspired repeated flashbacks to his playing days when he leapt off the bench during the game to dish out pats of support to the chests of various Spurs, as he so often did in the days he was faithfully flanked by Tony Parker and Manu Ginobili.

“When a young kid has Tim Duncan put his arm around him and guide them in a certain way, give them a piece of advice, that’s way more important than what I can do,” Popovich said.

Coaching full time, though, comes with ancillary demands, all of which Duncan happily avoided the past few seasons, when he routinely tutored San Antonio’s big men in the privacy of the Spurs’ practice facility. By agreeing to take a spot on Popovich’s bench, Duncan committed to wearing the sports coats he is known to loathe, to lengthy travel spells away from his children and to the prospect of headline-grabbing nights like Nov. 16 — when Duncan was thrust in the role of acting head coach, ahead of the more experienced Becky Hammon, after Popovich got ejected in an eventual loss to the Portland Trail Blazers.

The initial word was that Duncan, Hammon and the Spurs’ other top assistant, Will Hardy, were filling in by committee, but Popovich clarified postgame that Duncan had been in charge. In the first timeout after Pop’s ouster, it was indeed Duncan who addressed the Spurs.