By Boxing Day, Jose Mourinho had seen enough.

"It is another 's' goal," he said, after watching Manchester United draw 2-2 with Burnley at Old Trafford.

He wasn't talking about Steven Defour's free kick -- that, clearly, was a very good goal. His problem was with the first.

Johann Gudmundsson's free kick from the left was dropped into the far post where it bounced off a combination of Marcos Rojo, Romelu Lukaku and Ben Mee before Ashley Barnes smashed the loose ball into the net.

Three days earlier, Leicester had snatched a stoppage-time equaliser through Harry Maguire after United's defence -- albeit with Chris Smalling barely able to move because of injury -- had failed to deal with a long ball into the penalty area.

At the start of the month the Manchester derby had been lost after Manchester City scored twice from loose balls bouncing around the box -- one after a corner and the other after a free kick. The following weekend Gareth Barry scored an almost identical goal for West Brom.

And so, by Boxing Day, Mourinho's frustration was on the verge of boiling over.

"You are intelligent, you don't need me to tell the whole word, another 's' goal," he said.

"I don't need to say the other letters of the word. That's what we're having now, lots of 's' goals against."

Sources have told ESPN FC that Mourinho accepted privately that the problems were being caused, at least in part, by tired minds after nine games in December.

However, the week-long training camp in Dubai in January was as much a refresher course in organisation and basic defending as it was about recharging batteries.

It will be a concern, then, that those goals are back.

A long ball from the kickoff against Tottenham at Wembley gifted Christian Eriksen the chance to score after 11 seconds. Newcastle's goal on Sunday came from Jonjo Shelvey launching a free kick into United's penalty area.

Jose Mourinho is tired of watching his team concede goals from long balls or set pieces into the penalty area. Serena Taylor/Newcastle United via Getty Images

It is an odd quirk given that one of United's strengths this season was supposed to be the physicality of their players. Phil Jones, Smalling, Nemanja Matic, Paul Pogba and Lukaku started at St James' Park and at Wembley and all are 6-foot or taller.

Even stranger is that between the draw with Burnley and defeat to Tottenham, United went six straight games without conceding, their best run since keeping eight consecutive clean sheets in 2004-05. They have also got the best defensive record in the Premier League, having conceded 19 goals in 27 games.

Still, Mourinho's postmatch news conference at Newcastle gave away a lot about what he thinks is his team's biggest problem.

Though most of the questions were about why United had not scored or how to get more out of Pogba, the 55-year-old was more bothered about the other end of the pitch.

He even refused to criticise Smalling after the defender had needlessly dived in his own half to give Shelvey the free kick that ultimately won the game.

"I think we have to say the goal is a mistake," he said.

"A lateral free kick which travelled a long distance to the box. We lose the first ball in the air because we didn't compete in the air and then we lost the second ball on the floor.

"We train, we work, we organise, and the players have their individual jobs when they defend zonal. The players, they know there are responsibilities."

It was frighteningly similar to his assessment of Eriksen's goal for Spurs.

"It's a performance that starts with a ridiculous goal," he said.

"The first goal is an accumulation of mistakes. We lose the first ball in the air, we lose the second ball on the ground and it was a really, really bad goal."

Mourinho, at least, has a clear week of training before taking his team to Huddersfield for their FA Cup fifth round tie on Saturday evening.

He will remember, though, that in the 2-1 defeat at the John Smith's Stadium in October, Laurent Depoitre's goal came after Victor Lindelof had made a mess of goalkeeper Jonas Lossl's long punt up field. In a season of those goals, that might still be the worst.

United's struggles with aerial assaults have contributed to a run of just six wins from 12 Premier League games since the start of December. It has seen Manchester City double their lead at the top from eight points to 16. United have conceded 10 league goals in the same period of which seven have been from long balls or set pieces into the penalty area.

Of all the issues that have been raised in the inquest following defeat to Newcastle, it is the one that will annoy Mourinho the most.