Lucas returned from a brief benching by Coach Tom Izzo for a lack of leadership.
A 14-3 run capped by Raymar Morgan's 3-pointer gave MSU a 42-33 halftime lead, which swelled to 64-43 on Morgan's two free throws with 10:12 left.
From that point, Northwestern's deficit stayed in double digits.
Morgan scored 17 with five rebounds and five assists.
Durrell Summers had 17 points and seven rebounds, while Draymond Green added 13 with five rebounds and three assists for the Spartans (11-3, 1-0 Big 10), who have won two straight and six of seven.
John Shurna's 29 points and six rebounds led Northwestern.
Alex Marcotullio scored 16, and Michael Thompson and Drew Crawford scored eight points apiece for the Wildcats (10-3, 0-2), who have lost two straight following a nine-game winning streak.
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