First a general rule of thumb - If your best player at their position goes down your production is likely to be damaged. The way we worry about what will happen if so and so goes down is pathetic. We make it sound like no other team has any problems. Go ask a Lions fan who's backing up Calvin Johnson and see if he's excited by the prospect.
That being said here is what I think our back up receivers will work should something happen.
1. If Welker goes down, Edelman replaces him'
2. If Lloyd goes down, there are a couple of choices. The best physical fit would be Hernandez. Remember Lloyd is a deep threat, but not a speed threat. IIRC Hernandez is actually faster. In the short run, Hernandez would give us a receiver with the kind of size, quick feet, body control, and speed, to run all the routes that Lloyd will run.
However I think that in the long run the threat Hernandez presents by being moved all over the field in his current position is greater than the threat he would present on the outside as the Z. So if that were the case I'd send Edelman out there. He's faster but not as strong and a different kind of receiver. We'd have to run different patterns in that case.
There 's also the Gaffney option, but Gaffney isn't Lloyd either.
3. If Gronk goes down, the world as we know know it will soon cease to exist. But until that time Fells would back him up, He would be a capable replacement, but a step down in both blocking and receiving.
4. If Hernandez goes down Shiancoe would replace him. Also a capable replacement but a step down from Hernandez as a receiver
The problem with injuries is that you are never as good as you would be be with the "starter". But that doesn't mean that you can't get the job done with a combination of everyone who is left. Remember if one of the big 4 goes down, Brady will still have more elite targets than most NFL teams. It will be up to the coaches and Brady to elevate the replacement to put him into the the best situations possible to get the most out of his skills.
3.