He was a top class player in his day but not anymore. One of worst players on the field vs Ajax - sorry but we don't even play to his strengths which is baffling
And there in lies the crux in regards to his low amount of goals. Play to a players strength and he will perform well. He's still done well enough as a target man and he's been directly involved in a fair few league goals for us even if he hasn't scored that many. 4 assists and key passes to an additional 5 or so.
Even against Ajax he was very important by giving us the option of the long ball so as to not put too much pressure on Hugo, Sanchez and Wanyama having to carry the ball (the big reason why Ajax dominated so quickly. They pressed our most stressed players to hell).
He's getting old yes and he probably doesn't have multiple 90 minute matches per week in him anymore. But he's looked his best when off the bench, which is also when he has scored most of his goals.