Of with the coming of Delap and Pedro, Jackson should know that his time and place is up for Chelsea. Chelsea has really been patient enough with him to improve and he hasn't, it's time the club let him go even if it's for free, lol. As for Manchester United monitoring him, it will really be laughable because Jackson doesn't even fit into Chelsea teams that more stable than Manchester United. Seriously, I just doubt if actually Manchester United is even looking towards his direction.
You're very right, he was giving enough time to improve but he didn't, all those while he was on lapses was the time he was supposed to work on himself, with what we know sometimes players bad performance might be as a result of what they are going through emotionally but even at that such player expected to work on himself to overcome that very fast because football is group of people's business as I believe no one will want anyone to jeopardize with his business especially when the individual thats involved is been paid for the role of giving the club what they want, at this point it is obvious that Jackons now knows his stand in Chelsea for the reason you outlined Chelsea were patient enough to wait till this moment, so it is time for the needful to be done, if Manchester United need him fine for them but I don't think that guy got what they want at this moment but one thing in football is that a player might no perform well in club but we will be that the same player might do well in team B such is football.