Explain this move - candidates match 2007

Hello, I'm reading an annotated candidates match from 2007, Gelfand - Kasimdzhanov [1].

In move 16 the annotation says:

16.axb4 Bxb4 In case of 16...Qxb4 17.Ne1! Black is practically forced to sacrifice a pawn: 17...c5!? 18.Nxc5 Nxc5 19.dxc5 Rfc8 20.Nd3 Qb5, But the compensation may be insufficient.

I don't understand why after 17.Ne1! "black is practically forced to sacrifice a pawn".

Here is the full game with the variation on move 16...Qxb4

Any help is appreciated, thanks.

[1].http://www.chessgames.com/perl/chessgame?gid=1462032

My guess is white intends Nd3 with tempo then occupying c5 permanently locking in the passive b7 Bishop and the c-pawn will eventually fall anyways later in the game.

at least after Ne1..c5  blacks b7 Bishop is active like a positional sac

That does make sense. Thank you!