Deckard was a Replicant. Besides being the only healthy, strong, and perfect male in the film other then the known replicants, each replicant at some point in the film displayed glowing red eyes. Deckard’s eyes briefly glow red in a scene talking with red eyed Rachel in his apartment.
If Tyrell Corp was so worried about infiltrating replicants, I wondered why they just didn’t manufacture them with some obvious characteristic that would be identifiable on sight.
More human than human, that's our motto...