Two years ago, Illinois finished in the top 20 of tempo and had the 3rd most efficient offense in basketball.
Last year, Illinois finished in the bottom 20 of tempo and had the most efficient offense in basketball.
I'm sure Underwood will retool the offense again to fit his players, and they'll have one of the most efficient offenses in basketball again, regardless of who plays. Based on their preseason games, they'll play faster again.
As for Mirkovic, he's a top 5 player (and will be back sooner than later), but Illinois is also a very deep and old team, returning 6 guys from last year's team and adding 5 freshmen, at least 4 of which are better than expected from the jump, so that should allow them to weather the early storm of the schedule while Mirk gets back to 100%.