That is true the longer he is here the more ready he will be to play, however right now others are more ready than he is.
He has just completed his last season on November and is effectively going through a pre-season (with a group that isn't doing a pre-season).
Also it is possible for the manager to try something and for it not to work and require more time on the training pitch.
Using your logic Matos must have lost faith in Josh Key because he didnt play for a number of games, but that faith seems to have now been restored.