For six minutes and 46 seconds in the second quarter of Game 4, LeBron James wasn’t on the floor. His highness was fixed on the bench, the product of a career-first fourth foul in the first half. It left an opportunity ripe for the Celtics to pick apart a Cavaliers team without its leader on the floor and extend a double-digit lead to distances unknown.

They couldn’t do it.

Boston briefly took a 16-point lead in the second quarter, but squandered it. The Celtics watched the Cavaliers cut the deficit down to single digits. Before they knew it, the Celtics were only up 10 at the half.

Then, we know what happened.

Kyrie Irving exploded in the third quarter, scoring 21 points in the period on 90 percent shooting. LeBron returned, adding nine points and two assists of his own in the third, and the Cavaliers outscored the Celtics, 40-23, to take full command of Game 4 and the Eastern Conference Finals. Cleveland went on to win, 112-99, to take a 3-1 series lead with a potential closeout game in Boston on Thursday.

Why couldn’t the Celtics take advantage without James on the floor?

Take the best living basketball player off the floor, and his team is automatically going to get worse. That’s common sense, right? Well, it didn’t add up on Tuesday.

Obviously because Kyrie was so damn good, right?

Well, yes. Irving was magnificent, even before his astounding third quarter. With the Celtics in prime position to seal a second win at Quicken Loans Arena and tie the series at two apiece, the All-Star guard came alive.

In the final 5:11 of the first half, Irving scored 12 of the team’s 14 points on 4-of-6 shooting. Tristan Thompson was the only other player to score a basket during that stretch, while Kevin Love and J.R. Smith each missed shots.

In spite of a combination of defensive looks between Avery Bradley and Marcus Smart, Boston had no answer for one of the NBA’s most gifted scorers at the point guard position. Especially when he was slicing through the defense like he did on Tuesday.

In all, Irving finished Game 4 with a playoff career-high 42 points. That outburst started when James came off the floor in the second quarter.

Boston didn’t help itself by taking Al Horford off the floor.

When Brad Stevens took Horford off the floor with 5:45 left in the second quarter, Boston had a 13-point lead. When he came back in, the lead was down to 10, then dropped as low as six before Horford banked in a three to add some extra cushion.

Without Isaiah Thomas, Boston’s center is the team’s No. 1 option on offense. Avery Bradley is No. 2 and there’s little separation between options No. 3 and No. 5. Taking Horford off the floor while James was sitting with four fouls was equivalent to Boston shooting itself in the foot. Instead of pushing on the gas with James watching idly, the Celtics let up and took their best player off the floor.

Horford only had two fouls at the moment. He’d played 13 of 18 possible minutes when he was subbed out. Boston should have kept him in and run its offense through the versatile big man. Instead, it let up and forfeited an opportunity to extend its lead.

And the Celtics just don’t have enough firepower.

There’s a reason Boston was going hard after Paul George and/or Jimmy Butler at the trade deadline. They need another All-Star on the wing to truly compete with the Cavaliers for the Eastern Conference crown.

That much was proven on Tuesday when James stepped off the floor. Between Jae Crowder and Jaylen Brown, the small forward spot remained offensively obsolete with LeBron on the bench. It was as if they felt his presence from the sidelines.

Crowder went scoreless and the rookie Brown took only one shot during that stretch. No one was able to take advantage of Smith, an undoubtedly improved defender but not necessarily a stopper, either.

As a result, Cleveland broke even with Boston with James on the bench, scoring 14 points apiece as Irving (and Thompson) matched four different Celtics scorers.

We’ve learned a lot about Boston, not just in this series but in the entire postseason. Foremost, they’re a team that won’t lay down without a fight. Second, they’re one of the best offensive execution teams the league has.

But third, and maybe most important, they’ve got to get better. The Celtics couldn’t separate themselves from a Cavaliers team with James on the bench for almost seven minutes. And if they intend on shifting their fortunes next season, adding another All-Star on the perimeter is step one.

Unless this isn’t the Celtics’ championship window after all.