A Surprising Missed Winning Opportunity in Nakamura-Karjakin

In yesterday's rapid game between Nakamura and Karjakin, they played on until bare kings in an endgame.  It had seemed drawn for a long time, except for a moment of excitement when Karjakin offered a rook sacrifice on move 42 with 42...Rd2+!?.  Nakamura declined the offer, seeing that it would allow Karjakin to queen a pawn, and the game wound on to its drawn conclusion.

Karjakin, who was extremely short of time, did not notice that there was an even better version of his idea--one that would have won on the spot.  He could have played 

44...Rd3!!

 Nakamura-Karjakin after 42...Rd3!! (analysis diagram)

The point is to cover b3 so that White can't recapture with his king after 45...b3+.  If 45.Kxd3 b3! queens a pawn.  White's rook and knight are so out of position, in fact, that they are helpless to stop 45...b3 regardless of White's next move.