Not only was Deckard a replicant, he was a replicant uploaded with memories of Gaff, the character played by Edward James Olmos.

Gaff was an injured in the leg when responding to the original break-in at the Tyrell Corporation. One replicant got fried going through an electric fence, but a mistake in the script miscounted and there is one extra unexplained replicant that is never mentioned.

It is my firm belief that that captured replicant was programmed with Gaffs memories and tasked with finishing Gaffs mission, while Gaff himself had to act as a chaperone. And he's pretty pissed off about it because everyone seems to think this replicant can "do a man's work", as he says at the end of the film.