"There’s too many teams that got to lose," he said. "We dug ourselves our own hole."
Michigan State’s Bryn Forbes scored 17 and Eron Harris had 10 points.
The Badgers (16-10, 8-5) had won seven straight games dating to their victory Jan. 17, at home against Michigan State.
Wisconsin’s Ethan Happ scored 18 points and Bronson Koenig had 12 with a scoring surge late in the game.
"The guy we couldn’t stop with a fishnet was Happ," Izzo said.
Nigel Hayes was held to a season-low five points, 12 below his team-high scoring average, on 1-of-13 shooting for the Badgers.
"Hayes will never have another day like that," Izzo said.
Hayes scored 25 points in the previous meeting.
This time, he struggled to score and so did his teammates. Michigan State allowed the Badgers to barely make more than one-third of their shots.
"Our defense, I thought, was unbelievable at times," Izzo said.
The Valentine-led Spartans took control early and never gave up the lead.
"They were really dialed in from the start, focused in," Wisconsin coach Greg Gard said. "We dug ourselves a hole that we could never quite get out of."
Valentine had three assists and two points 3:13 into the game, helping Michigan State take a 10-3 lead. He assisted on or scored every basket for the Spartans when they built a 23-10 lead. Valentine had nine points and seven assists in 17 minutes, helping Michigan State to a 32-23 halftime lead.
"He makes them so much better because of how he raises up the level of play around him," Gard said. "It’s obvious that he’s healthy now."
Valentine missed four games earlier this season after having surgery on his left knee. In the 11 games since returning, he is averaging 21 points, 7.5 assists and 7.1 rebounds.
The Spartans started the second half strong, building a 13-point lead on Matt Costello’s dunk off an assist from Valentine. They didn’t have much trouble keeping a comfortable cushion against a team that was coming off a 13-point victory on the road against then-No. 2 Maryland, one of three ranked teams it beat during the seven-game winning streak.
Wisconsin: Koenig extended his streak with a 3-pointer to 37 games, but didn’t do much else for the Badgers until late in the game when it was out of reach. … Happ had a strong showing after the freshman was held to single digits in three of the previous four games, including scoring a total of six points combined in the previous two.
Michigan State: The Spartans hosted Wisconsin in their only game during an eight-day stretch that will give Izzo a chance to fine tune his team toward the end of the regular season. … Michigan State matched its lowest scoring total since Jan. 2 when it had 69 points in a win at Minnesota.