Avery Bradley of the Boston Celtics battles for the ball...

Avery Bradley of the Boston Celtics battles for the ball against J.R. Smith of the Knicks during Game 1 of the Eastern Conference Quarterfinals. (April 20, 2013) Credit: Jim McIsaac

As far as the Boston Celtics are concerned, they beat themselves. And that's especially frustrating.

In position to steal a critical postseason victory on the road, the Celtics admittedly went flat and got careless with the basketball in the second half. They scored only eight points in the fourth quarter en route to an 85-78 loss to the Knicks at the Garden in the opener of their best-of-seven first-round series.

Frustrated by his team's offensive ineptitude (only 25 second-half points) and penchant for turnovers (20), coach Doc Rivers sounded like a frustrated teacher eager to break out his red pen and scribble all over the repeated errors on his students' tests.

"I thought we stopped trusting a little bit offensively,'' Rivers said. "I thought each guy held the ball and tried to make their own play. I actually talked about that before the game. That's not who we are.''

Added Paul Pierce, "Obviously, these games are definitely frustrating when you know you have this opportunity right there in the grasp of your hand.''

The loss of star point guard Rajon Rondo to a torn ACL in late January continues to hover over this team. His absence was especially noticeable as the Celtics struggled to find any sort of offensive flow in the fourth quarter. They shot 3-for-11 and had eight turnovers. "Some were forced, some were boneheaded plays, but we need to have better execution,'' Pierce said.

Kevin Garnett said of the turnovers: "Playing on the road and against a good team that's energized, you don't help yourself. You don't give yourself the chance to win like that.''

Rivers thought his players often were too crowded and too idle on offense in the fourth quarter, and perhaps too content to simply wait around as spectators in hopes that Pierce would take over the game. That never happened. Pierce (21 points) scored only four in the fourth quarter.

"It seemed like we wanted Paul to win it for us,'' Rivers said. "We stood around and watched them play, and we can't do that.''

Pierce agreed. "Everybody needs to know where they're going to be at on the floor,'' he said. "Everybody has to get to their spots so when the fourth- quarter time comes, we all have to be on the same page. The games are too big for us not to be at that point.''

It also didn't help that Garnett looked like a shell of his usual high-energy self. A left foot injury forced Garnett to miss 12 of the Celtics' final 17 games, and the rust showed. He scored eight points and shot 4-for-12.

"He just didn't make shots,'' Rivers said. "We didn't get him the ball in the right spots. I thought we allowed them to guard Kevin with our spacing. I'm really disappointed with how we spaced the floor. And we can fix that.''

With Game 2 of the series looming Tuesday night, the Celtics are staring at the possibility of going home in a 2-0 hole unless they can figure out why they let a 70-63 lead in the third quarter turn into a most frustrating loss. "We just have to figure out a way to close out a quarter and close out the game,'' said Jeff Green, who had only six of his 26 points in the second half.