Just been away from the computer, hence not replying sooner.
I don't think it's a case of using Yaya minimally or thinking he doesn't give a shit etc
Yaya may well have played much better during 13/14, but even then, I do not remember him chasing or tackling back with great energy during that season, it was mainly because a lot of our key players, such as David Silva, Sergio and Nasri were also having good seasons, that we were able to paper over the defensive cracks in Yaya's game (usually by outscoring our opponents and being better at retaining possession in key midfield areas).
This season Yaya and a few others have simply not performed as well, this is what the manager should have realized sooner in the season and changed the tactics to better suit the players, 4-4-2 does not work with our crop of players, if they're not playing to their highest level.
As far as I'm concerned this season has gone poorly because of our stubborn manager, rather than because of one single player.