You can't permanently protect players on the practice squad. It's only a temporary thing you can do few times. He will be exposed to teams unless he's on the 53.
Risk losing Book, cut Seimian or keep 4 QBs. That's the only options.
Edit: I'm trying to remeber 100% if there is a weekly limit (can't find a source), but for sure the protection isn't permanent. After your game they become unprotected and can't be protected again until Tuesday. So they can be sniped on a weekly basis. Based on last years rules anyway.