When you say "... unless he shows that he can play the #1 ..." it sounds like you think 'the #1' is an actual position. It is not. In a 3 WR set the positions are typically X, Y, and Z (or split end, slot receiver, and flanker). The three positions are inherently different and require different skill sets, but on any given team any one of the three could be the best receiver on the team, i.e., their #1 receiver.
As far as teams having a 3rd corner to match up: First, that is only relevant in a man-to-man scheme. When teams go to their nickle zone defense the extra CB that comes in could be exploited by any of the receivers depending on where that guy is deployed in the defense and what particular play is called.
Second, it is not accurate in all cases to assume that in a man-to-man defense the CB covering he slot is the 3rd best CB on a team. Just looking at the Rams as an example: with Ramsey, Robey-Coleman, and Hill on the field at the same time I'd suggest that Hill is the 3rd best CB, yet it's Robey-Coleman who covers the slot in their man-to-man defense. Like slot receiver, the slot corner requires a specific skill set, one that makes him well-suited to cover slot receivers. It is a separate position from CBs who cover the outside guys, and teams specifically sign guys to play slot corner (rather than simply trot their 3rd best CB out there.) There is no particular reason to think that a slot corner is worse in coverage than the outside CBs simply because he's playing the slot.
AlbaNY_Ram
