Well ... not exactly. If/When Lampkin comes off the IL, he is on the 53 or he has to be waived.
A player has to pass through waivers before he can be placed on the Practice Squad.
The Rams waived Lampkin to get down to their 53. They could have placed him on the IL (like they did with Keir Thomas) but that would have put Lampkin out for the entire season (like Thomas).
Usually, when a player is waived with an injury (like Lampkin), the player is not claimed and reverts back to the team that waived him. Speculation is that what the Rams were hoping / planned; and then would have put Lampkin on the IL, with a designation to return.
However, the Eagles picked-up Lampkin; and he was on their 53-man roster. Couple days later, Philadelphia put Lampkin on the IL, with a designation to return.