"I've said this all along when everybody wanted to talk about offense and 3-point shooting, you've got to get stops to play that way," he said recently.
The Illini did well enough on defense to open the season 12-0, achieving a ranking as high as No. 10 at one point. The defense didn't become suspect, even in the initial loss to Missouri.
But something changed. Sometime over Christmas break, the Illini lost it's edge on the defensive end, a claim backed up by both statistics and Groce's criticism.
Since that loss to Missouri, a game in which the Tigers won despite being held to 39 percent shooting from the field, the Illini have gone 3-4, falling from the national polls and forced to look up in the Big Ten standings.
It all started in the game played against Auburn in the United Center on Dec. 29.
The Tigers, now 8-9, mounted a late charge, losing 81-79 in a game that shouldn't have been close at all.
Compare the numbers. Auburn scored 79 points and shot 48 percent from the field and 48 percent from the 3-point line. The team averages on the season read: 69 points, 43 percent (from the field), 34 percent (from 3).
The Tigers exceeded their season average in each category, but since Illinois won and improved to 13-1 the near upset was written off at the time as a game played amid distractions of the holiday season. From there the Illini lost four of the next six. A similar look at the stats point to defensive issues as the reason why.
Purdue for the season: 67 points, 42% (from the field), 34% (from 3).
Purdue against Illinois: 68 points, 41%, 38%
Minnesota for the season: 76 points, 47%, 36%
Minnesota against Illinois: 84 points, 53%, 60%
Wisconsin for the season: 69 points, 43%, 34%
Wisconsin against Illinois: 74 points, 49%, 44%
Northwestern for the season: 66 points, 42%, 37%
Northwestern against Illinois: 68 points, 47% 53%
A few notes here… In each loss, Illinois gave up more points than it's opponent average. Each team shot better from the 3-point line and three of the four (excluding Purdue) shot better from the field.
On the season, the four victorious teams are shooting 43.5 percent from the field and 36 percent from downtown. Yet against the Illini, two games of which were played at the Assembly Hall, these teams shot 48 percent overall and 49 percent from 3.
That's a large discrepancy, especially the increase in 3-point percentage.
After the Northwestern game, the third straight defeat by double-digits, Groce had seen enough and was quite clear in his comments that the defensive effort was not up to the expected standard.
"The disappointing thing for us is we had probably three or four things that we were trying to take away that we gave them, whether that was dribble handoffs or the back screens to begin the game, some of them to be honest with you by our better defenders," he said. "We weren't as locked in as we needed to be."
Ohio State for the season: 73 points, 46& (from the field), 37% (from 3)
Ohio State against Illinois: 55 points, 33%, 21%
Nebraska for the season: 58 points, 41%, 29%
Nebraska against Illinois: 51 points, 32%, 14%
Just like in the losses, the numbers show a difference in performance for teams against Illinois compared to the rest of the season. In these two cases though, the Illini held the Buckeyes and Cornhuskers below their season averages in points by forcing a drop in both overall shooting percentage and 3-point percentage.
Judging by the stats alone, it's not hard to figure out why four games were lost and two games were won.
The good news for Illini fans: the win against Nebraska was the most recent game played, providing hope that defensive struggles have passed the team by.
"It started on the defensive end," Groce told the Big Ten Network. "Our guys really competed. I thought it was the best we played defensively as a unit, and it was great to get that back."
The bad news: The Illini's next five opponents are the five best teams, conference record-wise, in the Big Ten.
"Our guys love that. They love challenges," Groce said.
The upcoming stretch will likely determine the ceiling of this Illinois team. And as the past six games indicate, the Illini will likely win or lose according to how they play on the defensive end of the court.