The gist of this, via Joe Freeman, is that Portland can sign a 16th player to the roster since at least four players have missed three or more games and aren’t coming back anytime soon.
If the Blazers exercise the exception, they would sign a player to a non-guaranteed contract and he would be eligible to remain with the team until one of their four most-recent injured players returns. Essentially, the Blazers would keep any player they sign until Rudy Fernandez (back) or Nicolas Batum (shoulder) returns.
So, cool. Yay. The Blazers can find a not-so-good big man for Joel-LaMarcus foul trouble insurance. It will not be a very good insurance plan, but it’s better than playing Nellieball.