I look at it like this:
Jack's play last season didn't make him an irreplaceable player, his talent will be hard to replacebut not impossible .
Mack was a force for the MBB team because of his enthusiasm and attitude, making the most out of his talent he could. But his game had a few holes even he couldn't overcome no matter how hard he tried to , so with a true PG in there to distribute the ball, RU's offense might improve enough not to miss Mack's O that much.
But like TDI said, need to see the team play a few games this season in order to get an idea if this year's play will be better than last year's.