According to Hleb (in an article published by Goal), Pep Guardiola never gave him an opportunity to play. I prefer to think that Hleb was a victim of a culture clash between himself and Barca.
Hleb was unprepared for the type of play that was expected from Barca since he was still in Gunner mode. The result were plays that were out of place in Barca with fans and coach eventually losing patience.
What eventually doomed Hleb at Barca was the presence of Lionel Messi and other stars that formed a dream team. The attempts to loan Hleb to other clubs such as Wolfsburg did not help him either.
Hleb and FC Barcelona parting by mutual agreement is the best for both. They were never a match in world football.
What will transpire?
My prediction for Hleb is that he is on the decline and that will be the case in Olympiakos. Hleb may stay one season at Olympiakos before retiring or signing with Anzhi Makhachkala.