Middlesbrough head coach Jonathan Woodgate slammed Marcus Browne after the substitute was sent off just 12 minutes after coming on in his team's 3-1 defeat at Swansea.
Browne appeared off the bench in the 51st minute and saw team-mate Marcus Tavernier cancel out Andre Ayew's opener to make it 1-1.
But in his short spell on the pitch, the 21-year-old was booked for diving as he appealed for a penalty and then received a straight red for a horror challenge on defender Mike van der Hoorn.
Middlesbrough ended the game with nine men as Paddy McNair was also sent off for an elbow on Matt Grimes. Woodgate's men have now received four red cards in their last eight games.
He said: "When we got back into it at 1-1 there was only one team that was going to go on and win the game.
"But then there is the decision from Marcus to make a horrific tackle. I absolutely can't condone that. It's a terrible tackle and it's a straight red card.
"He has let his team down. It's a red card all day long and I don't know what he's thinking. He had just been booked for simulation, so surely you have to think about that?
"Paddy's is a yellow card in my opinion, but the ill discipline from Marcus has cost us the game. It's a rash decision and a two-footed tackle.
"It's very frustrating. We will definitely appeal Paddy's and Marcus will definitely get fined."
Swansea went into the break 1-0 to the good thanks to Ayew's penalty which was his eighth goal of the season, converting after being fouled by Dael Fry.
Tavernier levelled the game up with a left-foot drive which beat Swansea goalkeeper Freddie Woodman at his near post before Browne lost his cool.
After being booked for diving in the box with Grimes close, he then went in over the ball on Van der Hoorn.
Swansea's man advantage allowed Ayew to make it 2-1 before Sam Surridge wrapped it up and there was still time for Middlesbrough to go down to nine as McNair departed.
Ayew was the game's outstanding performer as Swansea ended a run of six games without a win to move a point off the play-offs in a congested top half of the Championship.
Swansea head coach Steve Cooper said: "Andre has been brilliant. He's a class act both on and off the pitch.
"He delivers. His attitude and work ethic is right up there and he's a brilliant personality.
"Any player who does well at this level is going to create interest in moving up to the next level. We know Andre can play higher than this level, but he is a player who's giving more than 100 per cent.
"As long as he's here playing, it's for the better of the football club."
