Goals from Igor Thiago, Mikkel Damsgaard, and an own goal from Konstantinos Mavropanos earned all three points for Champions League hopefuls Brentford.
The East London club remain two points above the relegation zone but would fall back into it should Tottenham beat Aston Villa on Sunday.
Speaking to BBC Match of the Day after the game, Espirito Santo admitted that he believes the relegation battle “will go down to the wire.”
"I stand here honest and trying to speak to you about what I saw,” he said.
“Brentford is a very good team but the way we react and play the first half, hitting the post, having a goal disallowed for a tight margin, it gives us hope at half-time. But the penalty hurts us and after that it becomes an old situation with the rush.
"But it's up to us how we react to a tough loss today. (The penalty) It hurt us; it goes against what we wanted in the half-time talk. Unfortunately, we conceded from a penalty, which could have been avoided.
"It's up to us how we will react to the situation and noise. Let's stick together. We have three games to go. It will go down to the wire, the last game. We have to stick together."