The recent decline in the defensive effort of the No. 11 Arizona men’s basketball team is curious, but what’s even more dumbfounding is the disappearance of sophomore Nick Johnson from the court.

During the start of the golden months of the Wildcats’ undefeated non-conference play, Johnson was one of Arizona’s (23-5, 11-5 Pac-12) most consistent performers. He scored in bunches, passed like a point guard and — most importantly — played lockdown defense.

Ever since he picked up a flu virus at Washington State, his play has plummeted, and his on-court impact has been nonexistent at times. With a game against UCLA (21-7, 11-4) on Saturday at 7 p.m. in Pauley Pavilion, the Wildcats need the Johnson of old if they want to slow down the deadly efficient Bruins offense.

“He just has to get himself going,” said senior Solomon Hill after Wednesday’s 89-78 loss at USC. “We can try to do as much as we can as a team to get him going, but if we can’t get stops, we can’t get the ball, and it puts us in difficult situations.”

The dip in Johnson’s offensive production is no secret. After averaging 13.4 points per game during the Wildcats’ first 20 games, his output has been cut in half. He’s scoring just 6.5 points per game and making 36 percent of his shots, down from the 46.7 percent he shot prior to the slump.

Arizona can recover from his lack of offense, but it has been struggling to make up for his decline in defense.

Head coach Sean Miller designated Johnson as the defensive stopper at the start of the season, and the sophomore answered the call. He routinely guarded the opponent’s best guard and was dangerous in the passing lane, recording 2.3 steals per game, the top mark in the Pac-12.

Lately, his defense has just been ordinary. He was torched by Cal’s Allen Crabbe (31 points on 12-for-15 shooting), and he’s averaged a steal less (1.25) per game. With Arizona’s defense in disarray, the blame can’t all be pinned on Johnson. But as Arizona’s best perimeter stopper, he’s not helping.

Miller had two simple words to describe his guard’s recent play on defense: “Not good.”

His assignments aren’t getting any easier, either. UCLA guards Jordan Adams (15 points) and Larry Drew II (9 assists) both had strong games the last time the two sides met for an 84-73 Bruins victory in McKale Center.

And that wasn’t an aberration for UCLA.

The Bruins are tied for 22nd in the nation in points per game (76.0), are sixth in assists (17.1) and are 41st in field goal percentage (46.4). Along with the talented backcourt, they also have one of the best players in the Pac-12 in freshman Shabazz Muhammad, and have won five of their past six games, including a 79-74 overtime win against Arizona State.

All of this means the Wildcats will need the Johnson of the last meeting (23 points, 5 rebounds and 2 steals), not the Johnson of the last eight games.

“When you have a guy like that, you try to motivate him as much as possible,” Hill said. “Sometimes you just got to tell him that we need you out there. You can’t fade away … you’re more than a sophomore now; you’re like a junior to us.”

The gradual disappearance of Johnson at the end of the season isn’t exactly new territory. Last year, his scoring output dipped from 10 points per game to 6.1 over the final seven games. The only part of his game that has truly stayed intact is his passing ability.

The virus he picked up, plus natural wear and tear, might have Johnson feeling fatigued as the season nears its end. Miller had a simple answer for that: Give the young guard fewer minutes.

“I think one thing we can do to help his fatigue is not play him as much,” he said.

Johnson has played less than 30 minutes in a game only seven times this season. The first four came in blowouts in which Arizona won by 20 or more points. Another instance was Wednesday night’s game, in which Johnson played 23 minutes and was pulled off the court in favor of junior Jordin Mayes during important second-half minutes.

Johnson finished Wednesday’s game with four points, three assists and three turnovers.

The Trojans couldn’t be stopped on their way to a win with 61.1 percent shooting from the field. If Arizona wants to come away with a win in Los Angeles, it needs to hope Johnson is just in a funk he’ll break out of soon.

“I don’t think anything happened to his game,” senior Kevin Parrom said. “I think some guys hit slumps. He’s a great player. He’s going to get out of it, and hopefully he gets out of it soon. But I have faith in him. I think he’s going to be fine.”