To be a big disappointment, you first have to have high expectations for that player. What player has lofty expectations by the team and its fans?
Nick Foles? No. We just want him to be steady and not throw dumb interceptions.
Robinson? Yes. He needs to live up to his contract and take the next step in the development process.
Gurley? Yes. The back of the future as called out by Fisher.
Donald? Yes. A sophomore slump is a possibility, but I don't see it.
Quinn? Yes. With a healthy Long, Donald and Fairly, I see single coverage. Can you say mismatch?
Jenkins? Yes. We see the talent, but the mistakes are killing us.
Britt, Quick, Bailey? Maybe, but their performance is so reliant on QB play it has been hard to put all of the burden on them.
Austin? Yes. Traded up for him to be the spark plug of the offense. Has not yet happened.
Cook? Definitely. Another guy whose mistakes dramatically hurt the team...and his salary...woof
To me, those are the prospects for this list. I discount Foles because I don't expect him to be great. (I hope he is, but don't expect it.) I expect Cook, Austin and Jenkins to be great and to date they haven't been.