Kansas (23-8) will meet ninth-seeded Alabama-Birmingham, which upset top-seeded Kentucky, in the regional semifinals on Friday.
Pacific (25-8) had the nation's longest current winning streak at 16 games and upset Providence in the first round.
The Tigers was even with Kansas at 51-51 after a layup by Miah Davis with 10:49 left. But Michael Lee sparked a 15-4 run with a three-pointer and four free throws as Kansas opened a 66-55 lead with 6:48 left.
Simien had suffered a groin injury in the first half of Kansas' first-round win over Illinois-Chicago, but played 33 minutes on Sunday.