The experienced 6ft 7in striker was marked out the game by Nyatanga, then aged 18, during a Euro 2008 qualifying match for Wales against the Czech Republic.
Johnson has monitored the centre back's progress since then and, on Tuesday, his interest culminated in the Robins completing a deal for an undisclosed fee, believed to be in the region of £500,000.
"I was impressed as soon as I saw him play." Johnson told the Bristol Evening Post. "To be asked to play against someone like Jan Koller would scare most teenagers to death. But Lewin did a real job on him and I always remembered him after that."
He added: "He's still only 20, but he's already a seasoned Championship player. I was quite surprised when I heard he was available and we put a bid in as quickly as we could."
Nyatanga feels his move to City on a four-year deal will enable him to fulfil his potential.
The defender, who has already made 27 international appearances,
moved to Ashton Gate after 76 appearances for the Rams and loan spells at Sunderland and Barnsley.
"Because I came into the Derby team at such an early age and broke into the Wales set-up, I've been a victim of my own
success," he told the Evening Post.
"I've had to go out on loan a few times in recent seasons and it's been a bit frustrating for me. Potential is good, but only if you realise it. Moving here will help me achieve that.
"I'm still only 20 but I've got four years of first-team football behind me.
"I want to come here, improve my game even more and try to cement a first-team place."