The run was brilliant, stirring and nearly successful — the one where the Celtics scored 20 points in a 3:37 fourth-quarter span last night. However, the fact it came after they saw fit to get themselves 26 points in arrears to the Knicks helped complete the trip to a critical conclusion.

“The Run” is over.

As they left the floor last night, the Celtics had completed Year 6 of a three-year plan, having sipped every last bit of sunlight from their window of opportunity. Following the 88-80 loss, Doc Rivers put his own future with the club into play, and there are major concerns, if not doubts, as to whether Paul Pierce and Kevin Garnett will be back.

It should be pointed out that none of this is catching Danny Ainge off-guard.

If, in this last look, the Celts are to be praised for getting to a sixth game after falling behind 3-0, they must also be questioned for falling behind by 21 in Game 3 last Friday and coming out so mind-bendingly flat before their too-little, too-late tango last evening.

A 4-2 first-round defeat against the hardly mighty Knicks should remove any doubt that the Celtics need reconstructive surgery this offseason. Even if Pierce, Garnett and Rivers remain, this team needs more than the return of a healthy Rajon Rondo to again compete for a championship.

Ainge understands — but he also seems more than a little frustrated that the ability to do something about it is not entirely in his hands. And those who believe the president of basketball operations should simply wipe the roster clean and start over should probably be advised that he’s rung that doorbell on more than a few occasions.

As the Celtics began their offseason last night, there were no easy answers in sight.

“Well, you’re not going to find Paul Pierces and Kevin Garnetts on the free agent market,” Ainge told the Herald. “Those guys don’t exist — except for guys that are probably going to stay where they are.”

Chris Paul figures to remain a Clipper; Dwight Howard is odds-on to stay with the Lakers, though Dallas and Houston could make interesting pitches; and you’ll have to find cap space if you want to overpay Josh Smith.

That means the Celtics will have to get creative, but, again, they cannot do it alone.

“It’s always easy to have a plan,” Ainge said, “but the plan is really based on what your options are and what else is available.

“Last summer we had a lot of stuff going on. We weren’t sure if KG even wanted to play anymore. There were a lot of things riding on that. Once KG decided he wanted to play and come back to the Celtics, then our only option was to bring the players that we had under contract back. And we were able to do that, with the exception of Ray (Allen). So that was our only option once we decided we wanted KG.