Derrick Rose, the Knicks’ speedy reserve guard, poked loose what should have been a routine end-of-the-quarter possession by the Milwaukee Bucks, then dashed the other way with the ball for a last-second layup.

Rose’s crafty defensive play cut the Milwaukee lead to 16 at the end of the third quarter on Wednesday, and spurred the Knicks’ bench to begin swarming the Bucks on every possession. Soon, the Knicks were in the midst of a rousing comeback, tying the game in the fourth quarter after being 24 points down in the third.

Rose’s steal was emblematic of last year’s Knicks team: high-energy, displaying active hands and making nothing easy for the opposing team.

Except the Knicks couldn’t stop the 3-point shooting of Bucks guard Pat Connaughton, who hit four 3-pointers in the final quarter to keep the Knicks at bay. It was emblematic of this year’s Knicks team: unable to sustain defensive effort and punished from the perimeter by a hot-shooting guard.

The Knicks have made a habit of giving up big offensive nights to guards this season. On Wednesday, Connaughton hit seven 3s off the bench for 23 points, the most he’d scored in a game since Oct. 18, 2017. In the Knicks’ season opener last month, Jaylen Brown of the Boston Celtics went off for a career-high 46 points. The most surprising opponent performance of the year has been from Ricky Rubio, the backup point guard for the Cleveland Cavaliers, who scored 37 points off the bench on Sunday. Rubio hit eight of his nine 3s — this after being a poor shooter for most of his career. There was also the 36-point explosion from Toronto’s OG Anunoby earlier this month. Brown, Rubio and Anunoby’s performances were all career highs.

After a fast 5-1 start, the Knicks have come back down to earth, going 2-4 in their last six games. The slide has been primarily because of their defense. It’s the opposite problem of last year: They’ve scored at least 100 points in all but one game, and they are ranked fifth in the N.B.A. in offense. After Wednesday’s loss to the Bucks, the Knicks were the league’s 26th best team on defense — also known as its fifth worst. It’s a stark change from last year, when the Knicks were the fourth best defensive team.

The biggest shift has been in guarding the 3-point shot. Last season, the Knicks led the league in opponent 3-point percentage, meaning there was no team against whom it was more difficult to score from outside the perimeter. Now the Knicks are 26th.

The Knicks are also surrendering 41.7 deep shots a game — the most in the league. Last year, they were below league average, in the bottom 10. So the team is giving up more shots from 3 and has become worse at defending them, much to the chagrin of Coach Tom Thibodeau.The Bucks made 26 3-pointers against the Knicks, the most ever made against the team in franchise history.

When asked Wednesday about the kinds of 3-point shots that opposing offenses are getting against his team, Thibodeau was brusque: “Well, we don’t want to give up any shots.”

Thibodeau’s irritation is understandable. In a decades-long coaching career, Thibodeau’s defensive acumen has become his signature. During the 2007-8 season, he was the architect of the Celtics defense that won a championship. The defense was so good that Celtics power forward Kevin Garnett won the Defensive Player of the Year Award that season. In Thibodeau’s five seasons as head coach of the Chicago Bulls, the team led the league in defense twice and never finished lower than 11th. But in Minnesota, where Thibodeau spent two and a half seasons, the Timberwolves were one of the worst defensive teams in the league each season.

What’s odd about this current defensive spell is that the Knicks brought back most of their players from last year, when they were strong defensively. Plus, Mitchell Robinson, one of the league’s best rim protectors, missed most of last season but is healthy and starting.

The biggest change in the lineup has been the new starting backcourt of Kemba Walker and Evan Fournier, who are not known for their defense but have added some much-needed shooting punch.

Opposing guards are finding it easier to get by the Knicks’ starting guards and into the paint, especially off screen-and-rolls. This forces the Knicks’ interior defenders, like Robinson and Julius Randle, or Knicks guards to rotate over and help, allowing for open shots on the perimeter. Rubio, in particular, took advantage of this. When help wouldn’t come fast enough, he would take an open 3. If it did come, he used his foot speed to get around the defense and find easy looks for others.

Milwaukee’s Jrue Holiday and Giannis Antetokounmpo often were able to break down the Knicks’ perimeter defenses.

“I really can’t put a finger it,” Rose said. “I think it came from us not making shots on the other end then them putting pressure on us by using Giannis and Jrue in pick-and-roll to swing the ball to the corners. I think they hit like three straight 3s in the corner. They were just trying to expose us in different ways to make us help or over-help and spray out for 3s.”

In one sequence, RJ Barrett missed a layup and the Bucks immediately rebounded and ran the other way. Walker did not pick up Bucks guard Grayson Allen fast enough in transition. Allen didn’t have to work hard to get open and hit a 3 with Walker at least three feet away from him. Walker didn’t put a hand up.

On the Bucks' next score, Fournier was swallowed up by an Antetokounmpo screen, forcing Robinson to come help, leaving Bucks forward Bobby Portis open for 3.

These aren’t aberrations. There are other defensive issues for the Knicks that go beyond giving up jumpers. They have been below league average in giving up fast-break points. Last year, they were second. This is particularly problematic because the Knicks are playing at a slightly faster pace than they were last year, creating more opportunities for opponents to get out and run.

When the Celtics came to town, Brown routinely picked on Fournier, either in transition or when getting easily by him off screens. Often, Fournier would concede Brown’s shots rather than aggressively defend him, especially beyond the arc. Brown hit eight 3s in that game.

To add to their woes, the Knicks are taking more 3s than last season, but hitting them at a lower percentage. Missed jumpers often lead to long rebounds and make it easier for teams to start fast breaks.

The Knicks’ bench also has been defending better than the starters, generally, judging by defensive rating — a measure of how many points a team gives up per 100 possessions with those players on the court.

The good news is that this is all reversible. A team doesn’t forget how to play defense over one off-season and 12 games aren’t much to go on. And as the Knicks showed in the fourth quarter on Wednesday night, the team is capable of playing the stifling defense that became its identity last year. The chief problems the team has to solve are how to handle screeners and how to more aggressively chase open shooters. More effort will help, as Thibodeau has said.

But Thibodeau wants to solve the problem immediately, even if some may think the Knicks have played too few games to fret just yet.

“When it’s 10 games, you say you need 20. And when you get to 20, you say 30. And then once you get to 30, you say 40. And then before you know it, the season’s over,” Thibodeau said, referring to the concept with an unprintable word.