Has must-win time arrived for Illinois? John Groce would scoff at the notion. It’s only January, and he is, after all, a coach. His personal version of the ‘‘one game at a time’’ mentality espoused by every woman or man with a whistle is to compare each Big Ten game to a round of a heavyweight fight. An 18-round fight, mind you. They don’t make ’em like that anymore, but that’s beside the point. ‘‘We’ve talked all along about it being 18 rounds,’’ Groce said Tuesday, a day before the Illini (13-4, 2-2 Big Ten) will take on Purdue in Champaign. ‘‘This is Round 5 on Wednesday. We’ve got to be ready to go.’’ Rounds 3 and 4 were brutal on Illinois. First it was pummeled 95-70 at Wisconsin. Then it failed to connect at a pathetic rate in a 49-43 defeat at lowly Northwestern, shooting only 28.1 percent. Groce’s team entered those games with a shiny new Top 25 ranking and exited them looking an awful lot like a National Invitation Tournament squad. ‘‘Obviously, it was a tough week . . . [but] we’re smart guys,’’ senior forward Jon Ekey said. ‘‘We know, obviously, the last game doesn’t mean much for the next game.’’ Ah, but sometimes it does, and that’s why the game against Purdue is so important. A third consecutive loss — with No. 4 Michigan State coming to Champaign on Saturday — could set the season on a highly disappointing course. Groce put the two-game losing streak in perspective by pointing out how well the Illini played at both ends of the court in building a 13-2 record. Defensively, they were stout. Offensively, they were efficient, if not explosive, taking good care of the basketball and shooting 50 percent on two-point field goals. Against Wisconsin and Northwestern combined, Illinois made less than a third of its two-point shots. ‘‘Our body of work over 15 games, I’m going to choose that as the glass half-full rather than [choose] the span of two games,’’ Groce said. ‘‘We’ve got to knock a few more in. Our guys know that, and I’m confident that we will.’’