“We made a [different] sub and after that we played with one man less so that frustrates everyone. It’s obvious.
“When a player is down, you get information that he is okay to continue, then we make a sub and it turns out he can’t continue. We are all frustrated with that.”
The Portuguese boss added: “Football is emotions. It’s difficult to control. Especially when we had so much expectation and the fans were incredible.”
Forest looked on course to qualify for the Champions League at the start of April.
But a run of one win in their past six Premier League games has left Forest relying on the clubs around them doing them a favour in the final two matches of the campaign.
Nevertheless, Sunday’s result means Forest are assured of a top-seven finish that will end their 29-year wait for European football.
It was only three years ago that Forest were promoted after 23 years outside the top flight.
Chris Wood’s 20th goal of the season put them on course to beat Leicester after Morgan Gibbs-White earlier cancelled out Conor Coady’s opener.
But Buonanotte had the last word, with Marinakis making his feelings clear at the fulltime whistle.
Even if Forest win their last two league games, including a final-day meeting with Chelsea, it might not be enough for them to make the Champions League unless Aston Villa, Manchester City or Newcastle drop points along the way.
“Let’s go again; it’s not in our hands but it [the season] is not finished,” insisted Nuno.