The Cougars committed 10 turnovers despite the slow pace.
"They were able to really hurt us in transition," WSU coach Ken Bone said during a postgame radio interview. "I was more disappointed with that than anything."
The Cougs, stunningly, went without a made bucket for the last 9:09 of the game and still won.
USC (6-25, 1-17) missed its final three shots.
The Cougars, who have won three consecutive games against the Trojans and 14 of the last 19 in the series, split on the Los Angeles road trip after a 78-46 loss Thursday against UCLA.
"It was really important because losing to UCLA by that much was embarrassing," Motum said.
WSU finished tied for eighth place in the Pac-12 with Oregon State and will play the Beavers (17-13, 7-11) at noon Wednesday in the first round of the conference tournament at the Staples Center. It marks the first time the teams have met in the conference tournament since 1988.
Bone said his team will remain in Los Angeles until then.
"We want to rest our legs, but we don't want to sit in the hotel for four days either," he said.
The Cougars are the No. 8 seed by virtue of sweeping the Beavers this season. WSU has won four consecutive games in the series. OSU defeated both Colorado and Utah this week after losing their prior five contests.
"If there's any team in our conference that has not won as many games as people thought, it's them," Bone said. "They're very talented."
HE ALSO IS concerned about his own team. The Cougars were 6 of 11 from the stripe against the Trojans -- and made just 1 of 8 3-pointers.
"To compete in the Pac-12 Tournament, we're going to need to shoot better," Bone said.
He said he believed Motum, who scored a game-high 27 points, tired down the stretch as USC "crowded" him around the basket. The Trojans were able to get away with that as Motum had precious little help for a second consecutive game. With the exception of Motum, who is on pace to become only the third player in program history with 500 points and 200 rebounds in a season, no Cougars scored more than six points against either the Bruins or USC.
Bone said his team also struggled when the Trojans, who have been plagued by injuries throughout the season, went with a smaller lineup down the stretch. He said his team tried to counter that by doing the same, but there was some confusion on both ends about which players should be manning the post.
"They found ways to do it by being more aggressive," Bone said. "They scored in a variety of ways."
Despite the struggles down the stretch, Motum said he is not concerned about motivation.
"For some guys it's win and you don't play again," he said. "We'll bring it."