OKLAHOMA CITY -- When the doors to the Boston Celtics' locker room opened after Sunday's 99-96 loss to the Oklahoma City Thunder, the silence was deafening.

For the third time in seven days, Boston let a winnable game against a quality opponent slip away. Sunday's game followed an eerily similar script to Friday's loss to the visiting Toronto Raptors, with Boston fumbling away a double-digit lead with late-game lapses at both ends of the floor.

The Celtics are 8-7 this season in what the NBA defines as "clutch" games -- the score within five points in the final five minutes of the fourth quarter. That .533 winning percentage lands Boston in the middle of the NBA pack.

But a closer look reveals a more troubling trend, one that has been hammered home in the recent losses to the Thunder, Raptors and Houston Rockets. Boston's eight clutch wins have come against Brooklyn, Chicago, Indiana, Dallas, Detroit, Minnesota, Sacramento and Philadelphia -- opponents with a combined winning percentage of .344 (56-107). In addition to the three losses this week, Boston's clutch defeats also included Chicago, along with New Orleans, San Antonio and Detroit -- teams with a combined winning percentage of .595 (100-68).

That the Celtics have yet to produce a signature win is not news in these parts. It has been the dominant storyline in Boston this week -- well, besides a quick detour when some foolishly wondered if the Celtics were better without Isaiah Thomas, after the team put together a rare 48-minute effort in a lopsided win over the Orlando Magic on Wednesday.

The last two games have only hammered home just how much Boston misses Thomas (and he was technically injured in the second half of that Houston loss, too, after suffering the groin strain that has sidelined him since then). Thomas has been one of the league's top fourth-quarter scorers, and Boston has struggled without him to lean on the last two games.

After Sunday's loss, the Celtics tried their best to explain their late-game struggles against quality opponents.

"Words can’t even explain it with this one," said Marcus Smart, who was just short with a 3-pointer that could have forced overtime at the final buzzer Sunday. Smart was otherwise one of Boston's better players, putting up 11 points, nine assists and six rebounds while guarding Russell Westbrook for much of the night.

Added Smart: "It’s gotta stop. We play good for three quarters, first half, then we [lay] a goose egg in the fourth, and we let teams come back and take over the game."

Said the typically optimistic Avery Bradley: "We’re frustrated. We feel like we gave away two games in a row. Everybody’s a little down, but it’s our job to get back in the gym and fix all our mistakes and prepare for the next game."

Alas, no amount of practice or tape study will matter much to an antsy fan base that wants to see that practice work translate into wins. Getting Thomas back will certainly help Boston's cause, but coach Brad Stevens said Sunday that he had no idea if Thomas will be able to join the team before Wednesday's national TV tilt versus the San Antonio Spurs.

Asked how he makes sure his players don't get overly discouraged when close games have routinely gone against the Celtics, Stevens quibbled with the insinuation.

"They’ve gone our way most of the year. But they haven’t gone our way in the last couple of days, or in the last week," he said. "If you get too down or too discouraged 24 games in, you’re probably not built for this."

At this point, you can't help but wonder if the Celtics just need one game against a quality opponent to tip their way to sort of get the monkey off their back. How else can you explain the recent mental miscues like defensive miscommunication or Al Horford committing costly late-game turnovers?

"Even tonight, I feel like this could have been a great win for our team," Bradley said. "It kind of deflated us a little bit. But like I said, there’s a lot of games this season. We just have to move on to the next one and focus on practice. We’re going to get a good practice in on Tuesday and be prepared for the game Wednesday."

Still, the disappointment was palpable.

"It is frustrating, but we need to look at this film and have a good practice and get focused, because no one's going to feel bad for us," Horford said. "We need to pull out of this, and we need to stay together -- and we will."

The Celtics took no solace Sunday in ending Westbrook's triple-double streak and extending their own league-leading stretch of 226 regular-season games without allowing a player to reach those benchmarks. All that matters at the moment is the final result.

“Hopefully we’re learning. Hopefully we’re getting better," said Jae Crowder, who played only 27 minutes after fouling out in the fourth quarter. "I feel like in due time, we’ll get a big win like we need."