The Knicks opened the season playing surprisingly good defense. Team legend and MSG analyst Clyde Frazier constantly praised them on the network for playing “tenacious D.”

Tom Thibodeau’s team has strayed, however, from the principles that spurred its surprising 5-3 start while losing four consecutive games entering Friday’s visit to Cleveland.

In each of the four games, their opponents exceeded their season scoring average, and connected on at least 11 attempts from 3-point range.

That coincided with a shooting slump of their own, and the Knicks clearly don’t have the margin for error to revert to lackadaisical play in their own end.

“Disappointed that we could have played better, defensively, yes, but more determined to get it right,” Thibodeau said after Wednesday’s 116-109 loss to the Nets at the Garden. “We have to get back to practicing. I like the fight we showed at the end, but it was too late. We got in too big of a hole.

“We have to get back to work. Sometimes you go through stretches where you don’t have an opportunity to practice too much, so I think practice would be good for us.”

Thibodeau acknowledged after practice Thursday that he reinforced defensive principles at the start of the session, working on running the floor and getting back on defense, before switching off to other issues that have arisen during their four-game skid.

“We worked on our defense for a while and then covered some offensive things we wanted to work on,” Thibodeau said. “The main thing was trying to get up and down and practice out of the full court. I felt we needed work in that area, as well.

“I think the big challenge is when you have a number of areas that you’d like to shore up you have to prioritize. You can’t work on all of them every day. So you pick three or four things that you’d like to get into and improve upon and that’s what we did.”

During their first eight games, the Knicks held opponents to 43.4 percent shooting from the field and 29.8 percent from 3-point range. Those numbers have risen to 48.6 percent overall and 39.0 percent from long distance during the four-game losing streak.

The Nets still had Kevin Durant in their lineup Wednesday, but they were missing Kyrie Irving and the traded trio of Caris LeVert, Jarrett Allen and Taurean Prince while awaiting completion of the blockbuster deal for James Harden.

Defensively active starter Reggie Bullock has missed the past two games with a sore hip for the Knicks. He’s listed as questionable to return against the Cavaliers, whom the Knicks defeated, 95-86, on Dec. 29. Alec Burks (sprained ankle) is doubtful, while Frank Ntilikina remains out with a sprained right knee.

“We’ve got to stay with it,” said forward Julius Randle, who ranked third in the NBA through Wednesday in minutes per game (37.3). “It’s a long season, a lot of games to be played. It’s not at the end of the world after 12 games. Just stick with it, keep working how we’ve been working and things will turn it around.”