Comparing Keita to anyone is utterly pointless. He's not like Aquilani or Oxlade or anyone.
Personally, I just haven't seen anything that's giving the impression of a top PL midfielder there. Yes, its his first season, yes, he's had injuries, but he's part of a squad with great confidence, at a team that's top of the league and has a manager that is behind his players more than any other.
And yet Keita looks weak, slow, unfit, clueless. He's 2 yards off not just his own teammates but the opposition players too.
I think he needs to build his physique, train hard and get fit and up to speed. The disappointing thing is he should already be at that point.
If we're still seeing the same thing from him mid-way through next season then it's REALLY time to worry if he'll ever make it, but right now, I wouldn't put my money on it. These things happen. Not every player signed by LFC is going to be a guaranteed success and the same goes for other clubs, just look at Fred at United and Jorginho at Chelsea.