Swansea City manager Bob Bradley felt Middlesbrough's second goal came from a throw-in decision that wrongly went against his side.
Shortly after Alvaro Negredo opened the scoring, Adam Forshaw was released into the box by Antonio Barragan's quick throw - with Swans defender Jordi Amat clattering him and conceding a penalty.
Negredo made no mistake from 12 yards to double Boro's advantage, and a routine afternoon was rounded off by Marten de Roon's second-half strike.
After the game, Swansea boss Bradley highlighted the throw-in decision that ultimately led to Boro's second goal and felt his side were distracted by the officials' call.
"Our players thought it was our throw-in. Jay Fulton follows the ball over the line, and so we get caught a little bit when they throw in quickly. Of course, that led to the situation in the box.
"But you don't get anywhere complaining about results. We have to look at ourselves and I think it's the same story.
"We started OK, didn't make any big chances but I felt the energy and concentration was good. But their first (chance) at goal we go down.
"We've talked about our ability to hold together (after conceding), then the second goal comes in a strange way - a mix-up on whose throw it is, and we get caught. The next thing it is a penalty.
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"We put ourselves in a big hole. In the second half we had some moments of decent football but if we put ourselves in that position it's going to be difficult to get out.
"Of late, we've started games with confidence, playing positive football, not always creating enough but still with good intention on what we'll be about on the day.
"But our ability to avoid mistakes and not go down, or if we do go down to find a way back into the game, those are the areas that are costing us."
The result leaves Swansea rooted joint-bottom of the Premier League table, now six points adrift of Boro and set to spend Christmas Day propping up the division.
Heading into the game, the former USA coach - who took over at the Liberty Stadium in October and has overseen two wins in 10 - was the bookies' favourite to be the next Premier League manager to be sacked.
The Riverside rout will only shorten those odds further, but despite his side's dismal afternoon in the North-east , Bradley says he's still up for the relegation fight.
"I put pressure on myself to see if we can make the team stronger, fight for points every week and I'll continue to do that," he said.
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"I go in motivated into work every day, I like the group of players, I love the football club. It's a challenge - but I knew what I was getting into and I'm not backing down from it now.
"That's the challenge, the job. When a team has put itself in a tough spot then everyone has to stick together and fight the fight.
"That's what's left. We've said many times that we certainly think we can improve ourselves in January, but I've said the games between now and January are important.
"The next two at home - West Ham and Bournemouth - are important. But then we've talked about the possibility of improving in all areas.
"We talked about the importance of the Crystal Palace and Sunderland games (in recent weeks) but for the first time today we haven't taken care of business against a team right around us in the table.
"That will be important, and looking to our next two matches, winning home matches in our battle will be key."