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When the Patriots opened their season against the Tennessee Titans, they had five wide receivers on their 53-man roster: Welker, Brandon Lloyd, Julian Edelman, Matt Slater and Greg Salas.
Of those five, Salas has yet to dress for a game, while Slater is a receiver in name only—if the NFL allowed "Special Teams" as a true designation, that would be Slater's.
In other words, the Patriots have only had three wide receivers in games this season: Welker, Lloyd and Edelman.
Of those three, only Lloyd is currently signed for next season (through 2014, as are Slater and Salas). Edelman's rookie contract expires after this season.
It's quite possible that the Patriots need to know whether or not Edelman can be an effective replacement for Welker in 2013 and beyond. If so, then the Patriots can sign Edelman to an extension and allow Welker to walk, knowing that they have a cheaper replacement waiting in the wings (and an understudy to Edelman in Greg Salas). On the other hand, if Edelman can't handle the job, then that gives Welker more leverage in any negotiations that might take place.
So far, the results have been somewhat mixed. Through two games, Edelman has a total of six receptions for 57 yards. That may not seem like much, but that's already more than his entire 2011 output.
Moreover, as this CSNNE video discusses, Edelman spent time this summer working with Patriots QB Tom Brady, and he did earn one of the Patriots' coveted offseason workout awards.