No, it is relevant still, it may be a sunk cost, but it is still relevant if he remains on this roster. It is only irrelevant when determining whether to move on from him, because you cannot recover it, but if you considering further investing into him you damn sure better consider the costs you have already sunk into it. This type of thought is how people go broke investing.
I would not sign Amendola for that cost, and the reason is simple. I could sign Austin Collie for $765K non-guaranteed for one year, I think Collie offers basically the same skill set, same potential production and carries the same risk of injury. Amendola had one shot, he was signed by a marquee franchise, given the chance to replace the greatest slot receiver of his era, play with the GOAT QB, and was paid big money to do so, and he failed with his one chance. He showed the entire NFL that he is not the next Welker and is unable to remain on the football field. Now he is Austin Collie in my opinion, a player who can be productive as a fourth or fifth option but carries to much risk to justify a long-term contract.