Definitely a must-win. I’d actually argue Illinois needs to beat Wisconsin and Ohio State in the semifinal to guarantee a bid, or at the very least beat Wisconsin and lose a close one to OSU. They’re only something like 4-7 against RPI top-100 teams and while they do have impressive wins against MSU, WISC, Vandy, and Clemson on their resume they also have bad losses against Bradley and Utah and Northwestern as well. Plus they’ve lost 4 of 5 and the NCAA selection committee generally frowns upon bubble teams that “back-in.”
They need to win to get in, and pray for some help. They really have to hope for no upsets in the other conference tourneys where a team that otherwise would not get in the big dance wins their conference tourney and takes away one more at-large spot from the field. Tough position to be in but they made it this way with maddening play all season.