I think that Krejci is essential to the Bruins. As gifted as Seguin is, he isn't what I would want for a NHL center. Nothing against the kid, He is a rare and amazing talent and the Bruins need Seguin's skill set.
The reason I don't think he's an NHL center is because a true center needs to play both sides of the puck (see Bergeron & Kelly). They have to be the first to move the puck up from the Dman and then have to be the first forward back. What we saw out of Seguin was magnified because he was playing free of care (to some extent) because of Bergeron as well as Krejci.
Krejci, though did not have the best of seasons is a real NHL ready Centerman as is Bergeron and Kelly. They know how to play the position. Now I am in NO WAY saying that Seguin won't ever be a great NHL center, I'm just saying that there is a lot of growing up to do before hand. The greatest NHL Centers are Hockey-first mentalities (Crosby, Bergy, Steve Yzerman, Adam Oates, Mark Messier, Lemieux, Gretzky, and the list goes on) they stay away from the lime light and are natural leaders. Krejci and Bergy are the quietest leaders on the Bruins team. If you watch before a faceoff or between plays, both those guys are talking to someone not laughing about what happened the night before.
When it comes down to it to put Seguin as a 1st line center is incredibly irresponsible IMO. He's a great talent, but being on the wing will allow him to grow as a player as well as use his speed and stickhandling to it's full potential.
We need Krejci because besides Bergy and Kelly we have no one ready to be an NHL centerman. If we trade Krejci for Ryan, we need to trade a winger for a true Centerman. (example: Last year the NY Rangers got Richards, and they were the best team in the East with basically the same team plus him from the year before)
Don't hinder Seguin's development by adding a "unneeded pressure" to him. Let him grow as a person and more importantly as a player.
Just my 2 cents