I'm not sure where the idea comes from that McVay's offense can't feature a true #1 WR. Listed below are the top receivers (both receptions and yards) for the Redskins while McVay was on their staff. The leader in receptions averaged 80 catches and the yardage leaders averaged 880 yards.
(I don't know what the story was in 2012 but those numbers stick out like a sore thumb: excluding that season the averages are 85 catches for 922 yards.)
This, of course, doesn't indicate what Watkins would or wouldn't do in the future with the Rams, but to assume that his production would be limited by McVay's offense kind of ignores history.
2010: Sean McVay was the assistant tight ends coach for Washington. Santana Moss caught 93 passes for 1,115 yards.
2011: McVay was the tight ends coach for Washington. Jabar Gaffney caught 69 passes for 947 yards.
2012: McVay was the tight ends coach for Washington. Josh Morgan caught 48 passes, Pierre Garcon had 633 receiving yards.
2013: McVay was the tight ends coach for Washington. Pierre Garcon caught 113 passes for 1,346 yards.
2014: McVay was OC for Washington. Pierre Garcon caught 68 passes, DeSean Jackson had 1,169 receiving yards.
2015: McVay was OC for Washington. Jordan Reed caught 87 passes for 952 yards.
2016: McVay was OC for Washington. Pierre Garcon caught 79 passes for 1,041 yards. (DeSean Jackson also had 1,005 receiving yards.)
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