HOUSTON -- After four seasons of absolute lockdown defense, the Chicago Bulls have been stunningly mediocre this season. And Jimmy Butler believes he knows why.

Blame him.

"I think it starts with me, to tell you the truth,'' Butler said before Wednesday's 101-90 loss to the Houston Rockets. "I'm supposed to be this prime-time defender and I don't think I've been holding up my end of the bargain lately. So I think whenever I start kicking it up three, four notches on defense and not worry about offense as much, I think it'll all turn around.''

After ranking in the top 10 in defensive field goal percentage in each of coach Tom Thibodeau's first four seasons with the Bulls, including first once and second twice, Chicago entered Wednesday's game ranked 12th this season. Last year, the Bulls gave up 100 points only 16 times all season. This season, opponents were averaging 99.9 points a game, having reached 100 or more 25 times.

For the most part, the Bulls (30-20) have been able to withstand their defensive decline because they've become a much better offensive team. After finishing dead last in scoring last season at 93.7 points per game, they are averaging 102.2 points this year, good for ninth in the league.

But Butler, a second-team All-NBA defender last season, said the Bulls can't fool themselves into thinking good offense covers bad defense.

"That can't be a trade-off we're willing to make,'' he said.

Jimmy Butler has raised his scoring average to a team-high 20.5 points this season, but he blames himself for the Bulls' defensive problems. Joe Murphy/NBAE/Getty Images

Last week, after a 99-93 loss at Phoenix, Butler challenged the entire Bulls team to play better defense. But Wednesday, he was pointing the finger at himself.

The interesting thing about Butler's proclamation is that he is, by all accounts, having a breakout season. He has raised his scoring average by more than 7 points a game to a team-high 20.5 points and was named to the Eastern Conference All-Star team for the first time in his four-year career.

On top of that, he is still widely regarded as one of the league's top perimeter defenders and he routinely covers the opponent's best perimeter scorer.

But Butler admitted that his newfound offensive prowess has made it harder for him to buckle down defensively.

"You have to pick and choose your battles and save your energy for both ends of the floor now,'' said Butler, who leads the NBA at 39.8 minutes per game. "I'm not going to lie, I thought it was going to be easier than it is. But to go on one end and produce and then go on the other end and have to stop the best player on the opposing team is not always an easy task.''

One member of the Bulls' organization admitted that Butler's defensive tenacity has dropped a bit but said he still defends well in the fourth quarter. This person said the Bulls simply don't have the quality of defenders they used to have and added that injuries have led to a decline in Joakim Noah's play on the defensive end.

But Butler insists on looking in the mirror, and he promises he'll be better defensively over the second half of the season.

"I have to do it,'' he said. "That's what my team needs me for. So I'm going to do it.''