How would you know it hasn't? How would you know it isn't a case of it being a recurring theme? Or could possibly take a long while out injured to fix - something which isn't ideal for a developing player who is 23 this year?
They haven't said the manager doesn't rate him either...
To answer the first part of your question, if it was any of those, you would find him jet-setting off to the states where the club's preferred doctors and surgeons seem to work. But, assuming that this wasn't going to happen in the case of Ilori and the causes behind his injury woes have been kept a total secret, then you're right, there is no way that I could know whether or not he had a recurring problem that wasn't checked out by experts.
But then again neither could you. There is no way that you could say with any certainty that he had those issues. You could only hazard a guess and that would be totally worthless given that you're going to be unable to prove any of it because his injury problems are going to be kept totally secret.
As for the manager not rating him, here is a quote from Andy Hunter's article on the subject,
"However, Rodgers appears to have been unconvinced by the London-born defender and he was made available ahead of deadline day."
And here's Brendan's personal mouthpiece Mr. James Pearce,
"Many of those who have been shown the door were players like Ilori who were lauded by the transfer committee rather than being deals that Rodgers was the driving force behind.
Out of the eight players who arrived at the club last summer, four have already gone with three of them – Mario Balotelli, Javier Manquillo and Lazar Markovic – viewed as committee picks.
Of course Rodgers isn’t in the business of cutting off his nose to spite his face.
If he thought they were up to the task he would have kept them and played them. The bottom line is they, like Ilori, didn’t impress him, but it underlines the folly of signing players the manager isn’t 100% sold on."
Pretty damn explicit that Brendan didn't rate them. And that evidently includes Markovic and Manquillo as well.