It should have been easy to get him in and out.
Thomas was injured on the play before, so there was lots of time to get Williams in.
Then it takes time to unpile and spot the ball after a 3rd down play like that, with the possibility of even more time for a measurement. So as soon as the play ends Casey runs out on the field with a play ready. Especially since the line of scrimmage was right in front of the bench.
Having said that, I don’t think you sneak it with over a yard to go anyway. But at least with Williams in the defence has to be aware that he might. They knew Printers wouldn’t.