I have bolded the part where you are making assumptions. He was needed for fresh legs to bring on in the second half when the likes Petrov fitness wasn't up to it. If you couldn't see we needed fresh legs in the second half then well...
And that's what he did in the 13 matches he was a substitute in the latter 2/3 of the season.
But very rarely came on.
First 13:
7 wins , 3 ties, 3 losses
Last 12
4 wins, 4 ties, 4 losses
Faded, have a good day.