The Patriots are not letting PUP players DT Armond Armstead and WR Mark Harrison practice just yet. Head coach Bill Belichick says that he's not going to start their clocks in order to give them more time to strengthen.
Armstead was one of the headline signing this off-season as he was a top round talent who went to the CFL due to an injury. After starring in the league, he's come back and was expected to provide pass rushing depth next to nose tackle Vince Wilfork. Neither has really gone according to plan.
Harrison is a physical freak out of Rutgers (of course), who would provide valuable depth in case of an extended injury to Danny Amendola. They're entirely different players, but bringing in a player who's been in the system, despite not really practicing, is likely to provide less of a learning curve than bringing an outside player.
The Patriots have three weeks to activate these players to start practicing and, once that clock starts, they have three weeks to determine whether to keep the player activated or place them on the season-ending injured reserve.
I expect the Patriots to hold off activating these players until after the Jets game because the three week "test drive" practice clock will expire after the bye week, giving the team an extra two players in practice through the bye. After that time frame, I expect the Patriots to place Harrison on the IR to stash him away to next season, while Armstead would likely be activated, depending on his health.
In addition, as a not-so-fun fact, Brian McIntyre has pointed out that the Patriots have $20.6mm in cap dollars on the injured reserve, $4mm more than the next highest team in the NFL. That number represents roughly 1/6 of the allotted salary cap.