Gary Harris scored a game-high 19 points, but the Spartans fell to Illinois 53-46 on Saturday afternoon.
Michigan State had twice as many turnovers (six) as it had baskets (three) seven minutes into the game as it trailed 14-7.
Illinois might've had a lot to do with that.
"They defended very well, which they've been doing," Izzo said. "They turned us over a lot and turned those turnovers into touchdowns."
The Fighting Illini (17-12, 6-10 Big Ten) have held four consecutive opponents to fewer than 50 points for the first time since the 1947-48 season, according to STATS LLC.
Illinois has three straight victories for the first time since winning four in a row from Dec. 21 to Jan. 4. That successful stretch included a win over then-No. 23 Missouri.
"As soon as you act like you've arrived, you're going to fall pretty quickly," coach John Groce said.
Michigan State was at full strength for the first time in nearly two months, but simply wasn't good enough to beat a streaking team that seemed more inspired.
Branden Dawson returned from a broken hand and played for the first time since Jan. 21, when he helped the Spartans beat Indiana to improve to 18-1 overall and 7-0 in conference.
"I thought Dawson played phenomenally despite the 30-some-odd days he's missed," Izzo said.
Michigan State's Gary Harris had 19 points, and didn't get much help from his teammates.
The Spartans scored a season-low 46 points - five fewer than they had two weeks ago in a nine-point loss to Nebraska. They made 39 percent of their shots and had only one player with more than seven points.
Adreian Payne, who averages 16-plus points, had just four points to match a season low. Payne said he didn't expect to get double-teamed by the Illini as he did when he had the ball.
"It seemed like their No. 1 thing was trying to take me away," he said. "They did a good job of it."
Illinois, meanwhile, had a trio of double-digit scorers and made nearly 48 percent of its shots. Malcom Hill and Rayvonte Rice each had 10 points.
The Illini led by as much as 12 points in the first half before falling into a 28-all tie at halftime.
"We just wanted to go out there and set the tone early," said Tracy Abrams, who scored a team-high 12 points. "We wanted to start the game fighting, and that definitely had a big influence."
After giving up the first basket in the second half, the Illini didn't trail again.
Midway through the second half, Illinois took advantage of a call that went its way that enraged Izzo - and appeared to rattle his players.
Harris had a steal and a layup waved off because the officials decided a shot-clock violation happened before the turnover.
Izzo shouted at the officials about the call and pounded his fist on the scorer's table as he had some more choice words to say loudly.
"Our own people screwed up our own clock," he said.
Instead of the basket pulling the Spartans within a point, their deficit stayed at three and quickly grew to six points. The Illini went on to win at Michigan State for the first time since March 3, 2006, and to beat a ranked Big Ten team for the first time since Feb. 10, 2013, when they knocked off No. 18 Minnesota.
"We were responsive all night," Groce said.
And, the Spartans were not.
"We've got to start over," Payne said. "We've got to focus on what we can control and use the past to motivate us."