The team picked Watkins in the first round of this year's Draft to provide their quarterback with a new weapon.
However, the wide receiver told NFL Total Access: "It's not fully there. I mean, we've been connecting at mini-camp, and OTAs. But at the same time, I could be wide open, and if he don't feel it, or if I run it a different type of way, he won't throw the ball.
"So it really just got to come with him believing in me. I believe in him. He can throw just about any ball. So it just comes down to, we just need a lot of reps."
Manuel completed 1,972 passing yards in his rookie season with the Bills, but missed six games with a knee injury.