The other thing with Trestman is that he will understand the time it takes to adjust to switching leagues. It doesn't matter whether the player is going CFL>NFL or NFL>CFL, it takes time to learn the differences and the nuances of the new league. That process usually takes more than just a training camp to sort out. Trestman understands that as he lived it himself. He admitted that he found clock management in the CFL to be tough. He won't expect a CFL guy to be an instant hit in training camp. As long as the guy shows progress through the camp and that he's getting a grip on the challenges, then he'll probably be kept around as a backup or on the practice roster until he finds his feet and earns a starting spot.
As an example, with CFL tackles they'll have to learn to block without the benefit of a yard off the ball, plus they'll be handling DEs that are a bit bigger than the standard CFL variety. They won't adapt to that overnight. On the plus side, they'll probably have better range than the average NFL tackle as they're used to covering so much ground on both pass and run blocking. They're also used to pass blocking without the benefit of TE.