He wasnt good enough in LM against Chievo, Bate, Torino and other games he was subbed in as such. He is not a dribbler, so he is already up in the offensive half of left wing as a winger, he would mostly receive the ball while not running, and will be forced to dribble in short space while in stand still, or either pass back the ball.
In those above games, his choice has been to pass back the ball almost all times, because he isnt a dribbler if he doesnt have a momentum with him. Which he would if he played as an offensive LB/wingback, he would have space to run down the whole wing, and use his explosiveness to get into spaces opened up after linking up with the mids, and get past players on the run with accelaration and force (and he does this real well, seen it enough in azzurrini, and also against Milan...last season with Siena, he owned them then too).
The duels with Zambro was the perfect case (great ball by Marchisio, and he runs into that space, and keeps on going with momentum of his run), when there isnt that space to run into with the ball, he is very uneffective as a winger, and mainly just passes back to the mids (because he cant effectively dribble when in slower tempo, he is freezes up and plays it safe).
Thats his M.O which is quite obvious IMO, and its the M.O of a wingback, not a natural left winger IMO, he doesnt have the set abilities to make left wing his permanent choice of position. Is too limited and conservative to function real well as a permanent choice.
He is more of a Zambro then a Giggs/Ribery.