Jimmy is worth the #1 pick to the Celtics if they intend to go forward with their current roster and not start rebuilding. They are trying to get past the Cavs. Does another 20 year old, another lottery pick improve their chances because he'll be better than what they have to go toe to toe with Lebron? No they don't need more potential and athleticism. They need to add somebody to their current core of players who are coming into their own who can help face Lebron right now, not in 3 years when their players will be 3 years older, looking for their big payday contracts and they can't keep them all or some other team might be their competition.
If the Celtics lose to the Cavs this year then they are the ones who have too decide if Jimmy is the piece that they need to get to solidify their lineup to get to the finals. Whoever they might draft might help them win more regular season games in a year or two but he isn't going to change their chances of getting to the finals anytime soon.
I think Ainge is likely to go all in for the next 2-3 years by trading that pick and some of his players for either Jimmy or some other veteran who makes Boston a better playoff team. You just can't keep drafting and developing young players forever, when you've got a roster that can win games the next step is winning playoff games then winning the Finals. Continually adding more young players isn't how you win the Finals.