I wouldn't make this trade at all if I was Boston. Lupul may continue to have seasons like last year but I need to see it for another year. Krejci had a somewhat off year last year and is still better than Lupul imho.
there's a reason why Leafs are at the bottom while Bruins are at the top. Lupul's had one good/great season, Krecji been good for several years now and he's younger. Lupul+ but still wouldn't trade him for Lupul.
Big fan of Lupul, and I'd want to wait another year before deciding.... Although if I had to decide now, it would be Krejci > Lupul.
Lupul's biggest injury was a rare injury that is non-recurring (Blood infection) so I think his injury history is a little overblown (although there is some worries when it comes to his other injuries). Lupul has also shown a lot of potential since being drafted, putting together a great season with the Leafs this past year.
Personally, a deal around Lupul and Krejci intrigues me. Giving the Leafs a possible future first line of:
Kadri - Krejci - Kessel (Bad acronym though....)
That's a solid line, with Kadri's 2-way game improving, Krejci's good 2-way play, and both Kadri and Kessel's great offensive hockey IQ..... I can see that line doing very well in the offensive zone and alright-good in the defensive zone as well (something that would be nice to see...)
edit: JVR - Krejci - Kessel could be an alternative line