FWIW - Here's the roster cutdown timeline:
August 30 - Prior to 4:00 p.m., New York time, clubs must reduce rosters to a maximum of 53 players on the active/inactive List. Simultaneously with the cutdown to 53, clubs that have players in the categories of Active/Physically Unable to Perform or Active/Non-Football Injury or Illness must select one of the following options: place player on Reserve/Physically Unable to Perform or Reserve/Non-Football Injury or Illness, whichever is applicable; request waivers;terminate contract; trade contract; or continue to count the player on the active list.
August 31 -- Claiming period for players placed on waivers at the final roster reduction will expire at 12:00 noon, New York time. Beginning at 12:00 noon, New York time, clubs may establish a practice squad of eight players by signing free agents who do not have an accrued season of free agency credit or who were on the 46-player active list for less than nine regular season games during each of any accrued seasons. A player cannot serve on the practice squad for more than three seasons.
September 2 -- After 4:00 p.m., New York time, a club is permitted to place a player on Reserve/Injured as “Designated for Return.”
So if the Patriots decide to use the IR-DFR on Siliga or Chris Jones, that defensive tackle makes the 53-man roster on 8/30 - meaning that another player is waived or released. That cut player must clear waivers in order for him to be re-signed on 9/2 when the IR-DFR designation is used.