Wayne Rooney said he felt Derby had been denied a "clear goal" in their 2-1 defeat at Watford.
The Hornets were already 2-0 up when Colin Kazim-Richards headed a reply from a corner five minutes before half-time.
It was ruled out however – seemingly for a foul by Andre Wisdom on Watford goalkeeper Daniel Bachmann – but Rams manager Rooney was not convinced.
He said: "I think it's a clear goal, I'm still not sure why the referee has disallowed it.
"We can't dwell on it but maybe the keeper has had too much protection. I asked the fourth official and he couldn't give me an answer.
"The keeper came out the corner before and got a bit of a knock so maybe the referee was waiting to blow his whistle before an actual foul was committed."
Two goals in two first-half minutes from Joao Pedro and ex-Ram Will Hughes turned out to be just enough for promotion-chasing Watford to record their first ever home and away double at Derby's expense.
William Troost-Ekong's 77th-minute own goal gave Derby a lifeline and they bombarded the hosts for much of the remainder but could not force an equaliser.
Rooney had seen his side win five of the previous six matches but this defeat left them knowing their six-point cushion above the relegation places could be halved over the weekend.
He added: "I thought we deserved something out of the game. We had a 10-minute spell when we were sloppy and it has cost us in the end.
"In the second half we showed we were a good team that can cause problems to any opposition in this league.
"We showed a good reaction to going 2-0 down. I'm disappointed with the result but performance-wise it was OK."
The win moved Watford above Swansea into third, level on points with second-placed Brentford.
Hornets manager Xisco Munoz felt his side deserved to be rewarded for their efforts, even though they had certainly made hard work of it.
He said: "It was a brilliant first half, we played excellently. In the second half when they scored they gave us difficulties until the last minute but it was a good performance.
"In the last weeks no-one has said it would be easy. The Championship is a struggle and we have a lot of games.
"We can play better, that's for sure, but we have a good spirit. Now it is important to have a good recovery because we need to continue.
"It was difficult when I arrived because we would have been 12 points behind if we had lost against Norwich, but everyone has worked hard and played with passion. I feel very happy with everyone."
