By Matthew Geagan, CBS Boston

BOSTON (CBS) — If we’ve learned anything about the Celtics over the last few weeks, it’s that they are indeed human.

After their blistering start to the season, Boston is a pedestrian 6-4 over their last 10 games. The team that was making insane comebacks on a near-nightly basis is now coming up short in that department, and they have no one to blame but themselves. Wednesday night’s 90-89 loss to the Heat was one of their worst defeats of the season, as they let an undermanned Heat team without most of their starting five come in and steal a win on their home court.

Though they nearly erased an 11-point deficit over the last three minutes, and Kyrie Irving had a decent look at the end for a potential game-winner, the fact the Celtics let an eight-point halftime lead slip away should not be overlooked. It’s the first time all season the Celtics have lost when holding a lead at the break.

Boston never would have needed that furious run in the closing minutes had they done anything right in the third quarter. Miami outscored them 26-15 out of the half, and their overall play in over the final 24 minutes had guard Marcus Smart questioning their effort after the loss.

“If this isn’t a wake-up call for us, I don’t know what is,” said Smart. “It’s effort. We’re not talking on the defensive end, coming down and playing one-on-one ball. It really comes down to effort. They beat us to a couple of loose balls and wanted it more than us. They got rewarded tonight.”

Wednesday night was the second straight game the Celtics have gotten off to a fast start, only to find themselves behind in the closing minutes. They needed an improbable comeback to literally steal a win in Indiana on Monday, despite holding a 16-point advantage in the first half. Boston has been able to survive their tough stretches for much of the season, but when they don’t keep their foot on the gas, teams are now not only catching up but overtaking them.

The C’s late-game runs provide plenty of entertainment, but it doesn’t make up for their extended stints of ineffectiveness in the middle of games. The players feel comfortable turning to Brad Stevens during those trying times on the floor, but there is only so much the coach can do with his whiteboard.

“We can’t always rely on coach,” said Jayson Tatum. “There are five guys out there playing and you’ve got to communicate with each other.”

“I’m concerned with the way we stare at plays instead of moving the ball,” Stevens said of his team’s offense. “When we do that we’re not a very good team.”

There were certainly a few other factors on Wednesday night. The Celtics shot a disappointing 38 percent from the floor while old friend Kelly Olynyk couldn’t miss, leading the way for Miami with a career-high 32 points on 6-for-8 shooting from downtown. They had no idea what to do against a Miami zone in the second half, and their ball movement was often stagnant despite having 18 assists on their 30 made field goals. And it obviously didn’t help that Al Horford fouled out with over eight minutes left on a slew of extremely questionable calls.

But it was that kind of adversity that Boston was overcoming with ease earlier this season. They’re in a difficult stretch of the year, with Wednesday night being their 11th of 16 games in December. Stevens would love to get them back on the practice floor for some fine tuning, but that’s a luxury they won’t get until next week when they have consecutive off days for the first time in a month.

This is likely just a bump in the road for the Celtics, whose over-achievement last month set the bar pretty high. But when it’s a lack of effort that is causing that bump, it really is a wake-up call for a team trying to prove they’re one of the best in the league.