The Boston Celtics have seemingly followed every tough-to-swallow loss with an even tougher-to-swallow defeat.

It was the case Sunday as the Celtics squandered a once 18-point lead, which included a one-point lead (and the ball) with 8.5 seconds left, only to have the Oklahoma City Thunder escape TD Garden with a 105-104 victory.

It certainly doesn’t help that it was the Celtics’ fourth consecutive home loss and fourth in the last five games.

Simply, the Celtics are in a rut.

Over the prior two weeks, the C’s dropped what was then thought to be a ‘good loss’ to the Los Angeles Lakers and followed that with an overtime loss to the Houston Rockets, an overtime collapse to the Brooklyn Nets, a five-point defeat to the Utah Jazz and, most recently, the one-point loss to OKC.

And while there are too many things wrong to point to just one as the issue, the reality is the Celtics have been inconsistent. They have not been closing out quarters, and certainly not games, well. The things they did to help build Sunday’s 18-point lead, or even Tuesday’s once 21-point lead against the Brooklyn Nets, they are not doing regularly.

Head coach Brad Stevens knows it.

“I mean, it’s just kind of, it’s a sickening felling with this game and the Brooklyn game,” Stevens said, as seen on NBC Sports Boston’s postgame coverage. “This is a little bit different, but those are games you can get. And you got to figure out how to get them.

“This is part of navigating your way through this stuff. You can feel like you’re on top of the world one week and you can feel like the sky’s falling the next,” Stevens said. “And that’s the hardest part about the NBA, that’s just the way it goes. We’re in a way right now where we just don’t sustain it the way we have, and the way that we know we can.”

Celtics forward Gordon Hayward said much of the same.

“I think that’s how the NBA works,” Hayward said. “You go through these runs and stretches when you’re felling really good about yourself and then it’s like the world’s coming to an end too.

“As individuals, as a team, we got to try to not get too high on the highs or too low on the lows. And this is a low for us,” Hayward said. “We got to build ourselves, claw ourselves back out of it. I think we’ll find it again. We’re still the same team. We got to look each other up and try to find way to win basketball games again.”

Here are some other notes from Sunday’s Celtics-Thunder game:

— While it’s been a tough stretch for everyone on the Celtics, Sunday marked an especially tough ending for Kemba Walker. Again.

Walker was not only limited to 14 points on 4-for-14 from the floor, but the Celtics point guard also made one of the biggest (and not in a good way) plays of the game during the final seconds.

Walker coughed up a dribble as OKC’s Dennis Schroder stripped him in a double-team with teammate Chris Paul before Schroder finished a layup to put the Thunder up 105-104 with 8.5 seconds left. It was the game-winning basket.

And it comes just a few nights after Walker turned the ball over in crunch-time of regulation, allowing the Nets to gain possession after a jump ball and ultimately tie the game on the ensuing possession.

“It’s frustrating,” Walker said. “I mean that’s the second time it’s happened in three games. I just got to be better. You know, just find a way to hold on to the basketball and finish those games out.”

— Among the limited positives to come from the defeat was Hayward.

The Celtics forward was impressive in his first game back from two-game absence. He led the Celtics with a team-high 24 points on 9-for-19 from the floor. Hayward, who Stevens noted pregame would not be subject to a minutes restriction, played 38 of 48 minutes, as well, adding five rebounds and four assists to his stat sheet.

— The Celtics will look to put the devastating stretch behind them as they travel to Indiana to face the Pacers on Tuesday, with the game set for 7 p.m. ET.

Thumbnail photo via Bob DeChiara/USA TODAY Sports Images