The Packers have an open roster spot right now and they have not been getting a massive amount of production at the wide receiver position. As a fan of the team, I would be very enthusiastic about the idea of a reunion with Jordy Nelson that is similar to the Seahawks bringing back Marshawn Lynch. If all sides wanted it to happen, it could.
Jordy Nelson announced his retirement from the NFL earlier this season, and I previously thought that formally filing retirement papers would be the distinction as to whether he could return or not. If you'll recall, after Week 13 came and went, it was no longer a possibility for Rob Gronkowski to return to the Patriots.
However, that is a distinction not with retirement players but with the reserve/retirement list. You can read rules for that here. Because the Raiders cut Nelson in March, he is a free agent. Thus, like Lynch, he would be free to join the Packers if he wanted to and they wanted it to happen.
As to why this is a necessity, if you've watched the Packers play for even one quarter this season you've likely seen a route that had bad timing between Aaron Rodgers and the wide receiver. Geronimo Allison and Marquez Valdes-Scantling have had particular issues with production when targeted. Look at these stats for Allison:
Maybe this is totally irrational but I actually believe that Jordy Nelson could get on a private jet next Sunday, land in Green Bay 45 minutes before the game, get escorted to Lambeau, put on his uniform, go out there with zero practice, and have better timing with Rodgers than all of his receivers other than Davante Adams and possibly Allen Lazard and Jake Kumerow right now.
While Nelson has been adamant that he is enjoying his health and his retirement, the Packers should absolutely make the phone call and have him make a decision.