Had the Cardinals known the NFL would change its IR rules, the team might have considered waiting another week to make that move.
The new rule allows teams to activate one player from IR after the sixth week of the schedule. That one player can then play after the eighth week. But the rule only affects players placed on IR after the reduction to 53 players, which is set for Friday.
Players already on IR must miss the full season.
It's unclear whether Brown would have recovered from his torn triceps quickly enough to return during the season. Arizona would have had to carry him on its roster for a week at the expense of a healthy player.
Given the value of left tackles, however, that one-week roster sacrifice would have been worth the Cardinals' consideration. Brown was expected to miss at least three months.
The rules change was negotiated between the NFL and the NFL Players Association.
Update: The NFL has issued a news release with additional details.
The release reads, in part, "Since this rule is being adopted after the cutdown to 75 players, for 2012 only, the NFL and NFLPA have agreed to a transition procedure that permits clubs to return to their 53-player active list one player placed on Reserve/Injured since the start of preseason training camp. This selection of that player must be made no later than 9 p.m. ET tomorrow, Friday, August 31."