West Ham United are seven points from safety after a late Morgan Gibbs-White penalty - awarded by VAR - sent Nottingham Forest to a 2-1 win.
In what was being billed as a 'relegation cup final', West Ham looked to be in a fine position after Murillo's own goal and Cysensio Summerville's strike midway through the second half.
However, that Summerville goal was chalked off by VAR for an offside on new signing ValentÃn Castellanos and just four minutes later Forest equalised through Nicolas Dominguez in what was a huge swing.
VAR then stepped in to award Forest a penalty four minutes from time when Alphonse Areola caught Gibbs-White in the face attempting to punch clear a set-piece.
Nuno Espirito Santo was coming up against Forest for the first time since was sacked in September after his relationship with owner Evangelos Marinakis had broken down and is facing huge scrutiny as West Ham boss having lost five of their last six games.
Nuno said: "There's a big gap, but it's not over. We keep believing. We need a good run.
"It's not over. There's a lot of football to be played.
"It's a seven points gap. Everybody naturally will rule us out. But we don't allow this feeling to go inside the dressing room."
When asked about his future as West Ham boss, he replied: "It's not about me now. It's not about me. It's about how we react."
For current Forest boss Sean Dyche it was a welcome three points after a run of four straight defeats.
Nottingham Forest captain Morgan Gibbs-White speaking to Your Site on the penalty:
"I got whacked in the face. I didn't have a clue what was going on. Couldn't breathe. I'd just come to my senses when he said it was a penalty so there was no doubt in my mind I wanted to take it.
"I was practising it yesterday. It was kind of a safety one."
West Ham midfielder Tomas Soucek speaking to Your Site:
"It's a joke. I just saw it and we would have 20 penalties a game if it's like that. I came to the Premier League because I thought it was the toughest league in the world. We are all fighters, warriors but this looks more like basketball, you can't touch the player."
Nuno Espirito Santo has vowed he will keep on believing that West Ham can avoid relegation.
"I'll keep on going, keep believing, keep working with the boys to realise this is part of the game sometimes," he told Your Site.
"You have good moments, sometimes you have bad moments, but we have to be resilient enough to understand the situation. I'll make sure my boys believe it also."
Asked what his message to his players and fans was, Nuno said: "Keep believing, keep sticking together. It's not over yet. We'll keep on going.
"It's hard to ask [the fans] for anything. It's hard to explain. We understand their frustration, sadness, and we are all the same.
"What can we say? Keep helping us. Today they showed when the team gives, they give back the support that we need. The boys showed we are here to keep fighting."
And on whether he expected support from the West Ham board, he said: "Of course, we have to be in this together."
Nottingham Forest boss Sean Dyche said:
"I was delighted with the result. There was a lot of noise around the game, understandably so. It's about finding a way to win.
"We've obviously played better since I've been in the club, but today wasn't about that. Today was about a winning mentality and I thought we deserved it in the end. It's very pleasing about the mentality of the players and the direction we want to head in, because it can't always be beautiful.
"You've got to sometimes find different ways of winning. I think we've done that tonight."
