Diangana came off the bench away at Luton on Saturday and turned the game on its head, drilling in a long range effort before scoring a diving header.
The West Ham loanee, who was signed on deadline day after Slaven Bilic called up his former employers, staked a huge claim for a space in the team.
"I'm in shock!" he said. "It's an amazing feeling and definitely one of the best moments in my career.
"The first one, I had a one-v-one. He gave me a little bit of time to shift and shoot. I hoped it would go in and it did.
"The second one I was really happy with because I'm happy I got into that position. I'm usually out wide so to be in that position was really pleasing."
Diangana signed his first professional contract when Bilic was in charge of West Ham, and trained under him with the first team.
"He's a very nice person and a very good coach," said the 21-year-old. "To be able to communicate with him and speak to him about anything is great. He's always helping me with my game.
"He just said go on and be positive, be confident like I am in training and in games and to have an impact."
Derby were also interested in Diangana, but Bilic had tried to sign him all summer, and eventually got his man.
"Obviously I know Grady but not in the amount that some people think," said Bilic. "He trained with us when I was at West Ham but he was very young and I think even with David Moyes he didn't play.
"He emerged last season with [Manuel] Pellegrini. He played 17 games in the Premier League and they have [Felipe] Anderson, [Michail] Antonio, [Robert] Snodgrass and [Andriy] Yarmolenko – who got injured and that helped him – but they are top quality players.
"He got minutes there and started. I tried a few times in the beginning of the summer, but Manuel wasn't ready. They got [Pablo] Fornals in and Yarmolenko came back from injury, so they had options.
"It was a no-brainer for us. We like those kind of players, like Kyle [Edwards], Grady, [Matheus] Pereira, they're all young positive players, and take on people.
"That's great when they're surrounded by experienced players like [Charlie] Austin, [Kenneth] Zohore and [Jake] Livermore."