This is concerning, but given the lack of information it's a little early to get especially worked up about it.
These ACL, foot and ankle surgeries are concerning, but the fact these guys are suddenly on PUP shouldn't particularly elevate concerns. Getting worked up now might be a reflection of lack of appreciation for possible outcomes in the first place.
If one wants to get worked up based on incomplete information, then you have schadenfreude as a consolation prize.
The Pats put 7 guys on PUP, including Dion Lewis, Edelman, Amendola and Volmer, with their own ACL, foot and ankle issues mixed in.
As a side note, PUP is restricted to football injuries. USA Today has reported that Linsley's injury, undisclosed in the initial reports, is a hammy. You'd recon he tweeked it during offseason activities.
It's probably naive to thing these moves are purely medical with no consideration of the rules or the timing of opening day.
Keep in mind that if a player steps on the field for one preseason practice snap, he is disqualified for regular season PUP. While the injuries could be worse than we were led to believe, it could also be a case of conservative protection against a worst case scenario of being locked into IR or releasing a guy when PUP would be preferred where there is little downside to prevent forced IR at this juncture.
Further, if your thinking was to go slow with these guys as a precautionary measure in the first place, PUP enforces it rather than taking the risk of having some coach push a guy more than is warranted.
Further yet, it is inadvisable for a player to go half speed in a practice when everybody else is going full speed...that's a formula for further injury.
Which takes me back to the first line of this post: there is insufficient information to draw any conclusions.