They were gonna sign someone anyway. I wouldn't necessarily use the Barnes signing as an indicator that KD will be out for the remainder of the season. Barnes offers more than Calderon would have offered at this point. I'd say the KD injury just made them recognize that more and shift focus.
Barnes was available when they signed Calderon though, he's been on the market for more than a week now and they had the chance to sign either one of them. Immediately waiving a player to sign another who plays the position in which your star was hurt is telling, they still need another guard.