BOSTON -- Forget the rebounding differential or the free throw disparity, a highly entertaining Black Friday matinee between the Boston Celtics and San Antonio Spurs essentially boiled down to this: On maybe the game's most crucial possession, the veteran Spurs confidently ran a simple action that generated a wide-open look, while the young Celtics endured a monster communication lapse that spelled their demise.

In the final minute of a one-possession game, Patty Mills splashed a 3-pointer from the corner with five green jerseys cemented inside the paint, effectively securing San Antonio's 109-103 triumph over the Celtics at TD Garden.

The Celtics, winners of three straight and six of their previous eight entering Friday's game, were left to stew over all the little things that went wrong down the stretch. Yes, the Spurs dominated the glass (a 45-36 edge highlighted by old friend David Lee's 15-point, 12-rebound performance) and shot more free throws (a 25-9 edge in attempts), but the biggest takeaway should be that the Celtics still have strides to make in developing the sort of chemistry and cohesion that allows the NBA's truly elite teams to rise in the biggest moments.

"I was just talking with [Isaiah Thomas] about how [Spurs coach Gregg Popovich] knows what he wants to get, and those guys run it to a T," Celtics forward Jae Crowder said. "They have a lot of different playcalls in the fourth that they like to go to, and we knew them, but they just tried to pick us apart and just step into it. That’s one of the best teams that I ever saw in the fourth quarter."

Patty Mills (8) celebrates with Kawhi Leonard after draining a 3-pointer with 45 seconds left against the Celtics. AP Photo/Elise Amendola

The Mills play will be tough for Boston to watch during Saturday's film session. Mills set the play into motion with a little lob to LaMarcus Aldridge above the free throw line. After faking a handoff, Mills sprinted into the paint, with Manu Ginobili trailing close behind. Mills stopped at the charge circle, reversed to the corner and, while Ginobili failed to fully set the back screen on Thomas, raced free when Marcus Smart and Thomas had a miscommunication on the switch.

Thomas followed Ginobili to the opposite side of the floor, while Smart wandered into the secondary screen set by Kawhi Leonard on the wing. Aldridge fired to Mills, who didn't have a defender within 15 feet of him, before making it a six-point game with 44.1 seconds to play.

"Miscommunication," Thomas said when asked about the play. "I had thought Smart said switch, but those are little mistakes that we made that those type of guys in that organization are going to capitalize on."

Asked if teams feel like they almost have to be perfect down the stretch against teams like the Spurs, Thomas said, "Yeah, I mean, close to it. You just got to run your stuff, don't turn the ball over, get shots and box them out. We missed on a few opportunities there."

Al Horford said: "When you get in a back-and-forth with them early in the fourth, you just don’t want to do that against a team like that. I felt like we came out with great focus, great energy. Their bench just changed the game."

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The Celtics, who built as much as a 14-point first-quarter advantage, watched their lead erode, not because of Leonard (who had a quiet -- by his standards -- 25-points, 10-rebound, four-assist night, ) or Aldridge (10 points, four rebounds), but the play of a reserve trio in Mills, Lee, and rookie Davis Bertans who combined for 49 points on 18-of-25 shooting overall (Boston's entire bench combined for 30 points on 12-of-25 shooting).

Watching Lee dominate on the glass (seven offensive rebounds) will be tough for Celtics fans to stomach, especially after he arrived in Boston with a Larry O'Brien hangover from his summer of celebrating a championship victory with the Golden State Warriors. Lee morphed quickly from starting big man to squeezed out of the rotation in Boston's overstocked frontcourt. The Celtics eventually waived Lee and his $14.4 million contract in February.

"I’d love to be perfect and play great all the time," Lee said. "It wasn’t the case last year to start the year, and it wasn’t out of a lack of effort or disrespecting Boston or anything like that. It just worked out the way that it did.

"If I could go back and do it again, I would have probably tried to come in, in better shape, and that’s that. There’s nothing that I can do about it now but just try to make that same mistake in the future."

Lee said he has kept in touch with many of his former Boston teammates and maybe tried to win back some of their fans after Friday's game.

"Most importantly, [the Celtics] organization is in a great spot," Lee said. "Great group of guys and obviously well-coached. I think that they are one of the best teams we’ve played this year and should do great things."

The Celtics, healthier and playing better than at the start of this season, still lack a truly defining win. Victories on the road against potential East playoff foes like Charlotte and Detroit were nice, but Boston got blown out in the second half against Golden State and couldn't hold its early lead against San Antonio.

Asked to assess where his team is after Friday's game against the Spurs, Stevens said, "We are playing better. I don’t know exactly where to put [the Spurs] on the spectrum of where we ultimately need to be, but we’re getting better."

His players clearly yearn to play with the coolness and confidence the Spurs displayed late in a hostile environment.

"You have to be hitting on all cylinders. That’s from every guy who steps on the court, all playcalls, the whole scouting report -- everything has to be crisp, because those guys make you pay," Crowder said of the Spurs. "And when you have one lapse and one slipup like we did late in the game, they make you pay. So they teach us to be just more sound on both ends of the court and to take it all in."