If you're saying he might do well at Manchester United then that's totally fine but don't say Jackson did well at Chelsea because you clearly didn't know Jackson. Jackson was supposed to perform at Chelsea after his move from Villarreal but he failed to improve despite been giving several chances. Do you know that Jackson has been at Chelsea for 2 seasons? Do you know that in these two seasons his been their striker and has managed just 24 goals as a striker in 2 consecutive seasons. Scoring 14 goals in his first season in EPL and 10 league goals in the just concluded season and 10 assists in those two seasons combined. That's 24 goals in 65 appearances. Does this statistics look like a statistics of a player that performed? Like I said earlier, you clearly didn't know or watch Jackson play at Chelsea.
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.
Jackson was among the average players that was bought by the former coach of Chelsea and since that time, I haven't seen any significant impact that he has made in order for the club to achieve success. This is the main reason why I am against the swap of Gnarcho to Jackson because it's like moving from fry pan to fire. He has been given several opportunities to make a difference but he couldn't.