His Wildcats had led by 10 points late in the first half, and now they trailed by two.
But things got no better from there. In what could prove to be a damaging loss come NCAA tournament time, the Wildcats fell 57-56 before a sold-out crowd at Welsh-Ryan Arena.
The Wildcats finally did stage a rally after going down 55-47 with 1:43 to play.
The Wildcats tied it at 56-56 on a Drew Crawford tip-in with 30 seconds to play.
Illinois' Brandon Paul drove the lane and Crawford stepped in to take a charge, but none was called. Myke Henry got fouled on his tip-in attempt and made one free throw.
With six seconds left, Crawford drove it the length of the floor. Leonard swatted his game-winning shot attempt.
Northwestern went as Shurna did. He scored 17 first-half points, hitting a 3-pointer to give the Wildcats a 36-26 lead, their largest of the game.
But he didn't score again until 36 seconds remained in the game, a span of nearly 23 minutes.
Illinois' Brandon Paul did a terrific job defending him in the second half, as Shurna went just 1-for-7 after the break.
Drew Crawford had a tough night, scoring nine points on 3-for-11 shooting. Same for freshman point guard Dave Sobolewski, who didn't score until 37 minutes into the game and missed all four of his 3-point tries.