It's why Williams said Tuesday that rebounding is "probably the No. 1 thing we've got to work on" heading into Friday's NCAA opener against Providence in San Antonio.
"We just have to get back to understanding you have to box out every possession," sophomore point guard Marcus Paige said. "We can't just rely on size and length to get rebounds because at this point everybody wants to win the game. You've got to go make contact and try to box out better than we have been doing."
Rebounding has always been an emphasis for UNC in 11 years under Williams, who knows the Tar Heels (23-9) can't get off and running if they don't first snatch down a missed shot. They've regularly been among the ACC's best in rebounding margin and often among the national leaders, with this group ranking third in the ACC and 33rd nationally at plus-4.9 per game.
That's what makes the past two games so troubling.
First, the Tar Heels (23-9) managed a meager 20 rebounds in the loss at Duke's Cameron Indoor Stadium in the regular-season finale. It was the program's lowest total since managing 20 against North Carolina State in the 1987 ACC tournament championship game.
Then came the Pitt debacle in Greensboro. Less than a month after battling to a 40-all tie on the boards in the only regular-season meeting, the Panthers - well, Zanna, actually - terrorized the Tar Heels on the boards in an 80-75 loss.
Zanna finished with 21 rebounds, the most by an opponent against Williams' Tar Heels and only the second player to grab 20 since he took over in 2003-04, according to STATS LLC.
At one point, Zanna tipped in two straight offensive rebounds and had 17 rebounds to UNC's 16 with nearly 14 minutes still left.
"We weren't boxing out as well," Williams said. "We weren't going to the boards as well. We weren't getting off the box out when the other team had the inside position. It was just not doing a very good job."
It's all come after the Tar Heels, now the No. 6 seed in the East Regional, were outrebounded just once during a 12-game winning streak that erased an 0-3 ACC start. They have been outrebounded just eight times all year, losing five.
In those last two games combined, UNC has been outrebounded by 22, given up 28 offensive rebounds and 41 second-chance points. It's a major reason why the Tar Heels head into the NCAA tournament with two straight losses for the first time since 2003-04.
And that can't continue if the Tar Heels want to last beyond Friday's game, much less the weekend.
"When we won 12 in a row ... everybody was wondering, about are you going to get complacent?" Williams said. "Human nature is kids tend to slip if you allow them to slip. And I did a bad job because we've slipped in an area that I think is extremely important."