In the game book on NFL.com they have nextgen stats. One of them is average separation of the WRs on completions. You might want to check it out:
http://www.nfl.com/gamecenter/2017112301/2017/REG12/Chargers@Cowboys?icampaign=scoreStrip-globalNav-2017112301#menu=gameinfo|contentId:0ap3000000882073&tab=ngs
For the Chargers Allen was targeted 14 times and had 11 catches, his average separation was 2.17 yards. That is well below the NFL average of 2.74 yards. For the Cowboys TWill was targeted 5 times with 4 catches, his separation was 3.41 yards. Overall the Chargers had no more separation than our WRs on completions.
Edit: And they have those stats for every game. So I also know that Alshon Jeffrey for the Eagles was targeted 7 times and had 4 catches vs us. On those catches he averaged a whopping 1.88 yards of separation.
NFL QBs make tight throws, a LOT. They complete them a lot. And there is frequently very little separation. That's pretty much what it means to throw a receiver open.