The last time I checked both guys (Pierce and KG) Statistically had a similar level of impact, only on opposite ends of the floor.
Basically our defense got about 13 points worse (allowed 13 PPP more) when KG sat down, and our offense got about 13 points wirse (scored 13 PPP less) when Pierce sat down. At the time those two guys had the highest individual impacts on the team with KG's net impact being the stronger of the two.
But IMHO KG is more imporant to the Celtics right now because Jeff Green has emerged as a guy we can lean on to provide offense (20.5 PPG and 52% FG in 17 regualr season starts, 20.3 PPG in 6 playoff starts) - that makes Pierce far more expendable because now we have another guy who can score in bunches when Pierce is not on the court.
On the other hand the KG issues still remains. When KG goes to the bench we have Wilcox, Randolf, White, Sullinger, Bass who can come in at center. Sullinger and Bass are the only guys in that group who have any ability in terms of low post defense, but they are both way too short for the center spot. No matter who we put in, we still get slaughtered any time KG sits.
To add to all of this, Pierce is easier to trade right now. His $16M contact can be bought out for $5M if another team acquires him be a certain date, and that means he's a viable trade target not only for good teams who want to make a title run, but also for any team who wants to buy $16M of cap space for $5M.
Then add to this the fact that Pierce and Green don't really work that well together. Playing both guys means you either need to play Pierce at SG or play Green at PF - both options are a compromise and means we aren't making optimal use of one of those guys. We could bring one off the bench but Green clearly plays FAR better as a starter, and Pierce would not likely accept a bench role. That means one of the guys will probably go. Pierce is 35 years old, only has prboably 2 years left in the league, and is due $16M next season. Jeff is only 27 and has another 6-8 years left in him, and he's only due $9M next season. From a GM's point of view the decision is somewhat if a no brainer.
KG is replacable if (and only if) we can find some way to pick up a starting calibre center who can give us at least 30 productive minutes per night. I'm thinking Marcin Gortat, Emeka Okafor, Demarcus Cousins, Al Jefferson...even DeAndre Jordan. Jordan wouldn't come close to replacing what KG gives us, but he would at least fill part of the hole that would be created if KG left. If that's not possible then we really have no other true center on the roster, so KG remains a critical part of the team.