The movie pointed out several times that the Batty crew were the most advanced line of replicants the Tyrell corporation had ever produced. If Deckard was a replicant maybe he was an earlier model. That would also explain why he wasn't as strong as the Batty crew. I have absolutely nothing to base this on but it would fill the plot hole.
Reading the comments, some were also speculating that Deckard was a more advanced model of replicant. Why? While not stronger, he apparently lived longer and was more cleaver than Batty & crew. But who knows?
Like I said, I never even heard anyone suggest that Deckard might be replicant until 2+ decades after the original came out.