That's an interesting rule
............The team announced Monday that
McCaffrey will be placed on injured reserve, ending his season. ...........McCaffrey missed five games earlier in the season due to a hamstring injury that sent him to IR.
By rule, a player cannot return twice from IR in a single season..........
The IR has loosened up quite a bit over the years.
Pre-2012 a player on IR couldn't return at all during the season. Then in 2012 it was one player per team but only after 6 weeks out. Then in 2017 two players per team were allowed to return from IR but after 8 weeks out; in 2018 it changed to after 8 games to allow for bye weeks.
Due to COVID there was a major change for 2020: unlimited number of players per team could return from IR and after only 3 weeks out
I assume the "twice on IR then no return" rule came in last year with the "after three weeks out" rule. It wouldn't have been applicable under an "eight weeks out" rule.
Of course, it is a tad more complicated than that. e.g. a player on IR is not included in the 53-man active roster but is included in the 90-man team roster.
Why the "no return" rule?
I've no real idea but I could guess. It would, theoretically, be possible to manipulate your active roster by putting a player on-off IR.