You have a valid point here though. As a striker, you are expected to score a minimum of 30 goals and have 10+ assist in one season. Anything less than that is not good for the team if they plan on winning the trophies. Also, Chelsea is a big club, if a player is not doing well, they have to call the shot. Just like you said Jackson has been in Chelsea for 2 seasons. And his performance across all competition is very poor. Now Chelsea has signed Delap and Jao Pedro I dont see any need for him.
Every person, every worker has goals to achieve.
The coach's goal is to make winning strategies, the goalkeeper must save goals otherwise.
The striker must score goals, if he doesn't we can say he's absolutely useless.
So, to the wise, a few words.