They can pluck him as long as he is signed to their 53 man roster.

Waivers are a lot different than signing somebody from a practice squad. What happens is when a low-experience player is released by a team he is “waived”. He then goes through a 24 or 48 hour period (not sure which) in which his is “up for grabs” by any other team in the league based on (I think) draft order. When a team claims a player off waivers the original contract is not voided, it is transferred unaltered in any way except for the “owner”, so the new owner must now pay the player exactly what his original contract stipulated for the balance of his contract terms (unless, of course, they waive him again before the end of it). So if a player has a third round contract and they are waived, the signing bonus (if any) would have already been paid by the original team but the remaining money, escalators, roster bonuses, etc. would now be the responsibility of the new team. But on the other hand, the player who got waived has zero say-so in where he goes. If a team like the Lions puts in a waiver claim on that player, they are going to get him and the player can either hold out or go play for the Lions and that’s basically all he can do.

This is monumentally different from an experienced player, who must have his contract “terminated”. In the latter case the player is an instant free agent, free to sign with any other team (even the one who just terminated him in some wacko circumstances) whenever and wherever he wants, but he has freedom to refuse any offer another team makes.

Once a player clears waivers, then their old contract is finally terminated. At that point they can be signed to a practice squad. This is a new contract and is non-exclusive. The player is essentially being paid to help the team practice for upcoming opponents and he can hope that he’ll eventually get good enough to play “for real” on Sundays. But he remains a free agent and any team can approach him at any time with a contract and he has the option (just like a veteran free agent) to accept it or refuse it. The new contract must be to the 53 man roster of course, and the new team has to keep the player on that 53 man roster for at least three weeks. But other than the above and the CBA protections on injuries and whatnot a player on the practice squad is in just about every other way basically an unemployed player able to be cut loose at a moment’s notice with no penalty for the team cutting him loose.