"We saw Jeff George take over," Illini Coach John Mackovic said. "I think he's shown he's one of those exceptional quarterbacks."
The winning pass came after John Langeloh kicked a 21-yard field goal that put Michigan State ahead with 4:18 remaining.
The Spartans appeared to have the game wrapped up after stopping Illinois' next drive and forcing a punt. But on the first play after the punt, Highland Hickson fumbled, and defensive back Quintin Parker recovered, setting up the winning drive.
Illinois went 35 yards in three plays, taking only 28 seconds.
"It was a tough one to lose," Michigan State Coach George Perles said. "I guess you have to give them credit for not giving up and for causing a fumble and putting it in the end zone."
Of the fumble, Perles said: "I would probably have to call the same play again. Obviously it was the turning point of the game."
After the fumble, George completed two quick passes, one to running back Kameno Bell and one to tight end Jeff Finke, each for a gain of 13 yards, before finding Bellamy for the game-winner.
Langeloh kicked his field goal after Michigan State, which hadn't mounted much of an offense all day, moved 91 yards in 16 plays.
Neither team was able to generate much of an offense, with stubborn defenses and offensive miscues combining to stop drives and force both sides into exchanging punts.
Illinois is 5-1 overall and 3-0 in Big Ten play. Michigan is 2-4 and 1-2.
On the game's first possession, Illinois went 75 yards in eight plays to score, with George rolling to the right and hitting tight end Dan Donovan from one yard away.
The touchdown was set up by a 53-yard pass play from George to Bellamy on the second play of the game. With plenty of time to throw, George hit Bellamy over the middle, and Bellamy raced to the Michigan State 20 before being forced out of bounds.
Michigan State tied the score in the opening moments of the third quarter. Flanker Courtney Hawkins broke loose up the middle with the half's opening kickoff and ran 85 yards to the Illinois seven. Two plays later, tailback Tico Duckett scored from the one.