Irving is really slow
Really slow is a bit much, but he isn't super fast. But sort of the opposite of Denis, he has decent lateral quickness. His biggest problem is that he hasn't completely adjusted to the speed of the game, ie. when an offensive player goes north-south at full speed against him, he doesn't turn and run with the guy soon enough and gets beat. Sort of like a defensive back's cushion in football. Fortunately when this happens he usually fouls before the guy can get to the rim for a lay-up.
That said, he is big enough and physical enough to play pretty solid defense in half court sets b/c he doesn't get knocked off nearly as easy by ball screens, etc. like Russell and McGruder do, and this is a big reason he plays. Also, he handles the ball the best of the 3 FR guards, so he gets the job of spelling Denis/Jake when they get tired or in foul trouble, and the fact that he has gotten double digit minutes in every game during our 7 game winning streak only supports that.
Irving likely will never be a star and doesn't have near the upside of someone like McGruder, but he does the things that Frank wants him to do right now, and is the best fit for the minutes available. JHR is in a similar state, he does the things Frank needs right now, while clearly Judge has more upside. But Frank isn't going to play potential right now, he's going to play who he feels best helps the team win. Against Missouri we needed our best defensive players on the floor, and of the FR Irving and JHR provided that. Wed night we may need more offense, so Judge and McGruder may play more like they did the first time.
It really isn't too difficult to see how Frank is using and has used the FR this year and I'm sure the players understand it as well.
