Paddy McNair walked off the pitch at the St Mary's Stadium on Monday night dejected and disappointed before slumping into his seat on the bench.
Only 38 minutes had been played, but such was the nature of both the Ballyclare-born youngster's performance and that of the Manchester United defence he was part of, that something had to give for manager Louis van Gaal.
The Northern Ireland under-21 international lacked confidence, said Van Gaal. Taking him off before half time won't have done a lot to boost that, but former Northern Ireland international defender John O'Neill, who played at the 1986 World Cup finals in Mexico, believes that it's all part of the teenager's learning process and that he will be stronger for the experience.
"I sympathise with Paddy. It happened to me once in my career, but I was allowed to go on for another 15 minutes being rubbish before I was substituted," said O'Neill.
"It is very embarrassing when it happens, obviously in front of the spectators who are there and the audience on TV.
"Paddy wasn't having the best of games. You may criticise Louis van Gaal for getting the tactics wrong, but at the same time you have to admire him for changing it. United went on to win and now they are third in the table.
"It's a challenge for the lad and he has to accept it and look at what he did wrong and obviously improve on that and come back even stronger.
"The lad is only 19, yet he is playing for Manchester United, one of the biggest clubs in the world. He's got to remain confident in his own ability, he's there on merit and you're not going to play brilliantly in every game.
"He's had one poor game, but that's what happens when you are with Manchester United. Everything gets blown out of proportion. The fact that he is playing there at 19, he'll overcome this.
"I just keep my fingers crossed that somewhere down the line we'll have McNair and Jonny Evans as the two centre-halves for Manchester United.
"When the manager comes out and tells you straight that you've been rubbish you know that the next time he comes out and says you were brilliant that he means it and players will respect that."
Evans had come on after 18 minutes, when Chris Smalling picked up an injury. It was the first time that the two Northern Ireland boys had played together in the United first team, but it lasted just 21 minutes.
McNair had been playing well in the centre of a back four, but Van Gaal changed the system and deployed three centre-halves against Southampton and the youngster looked anything but comfortable in that set-up.
The question is, should Van Gaal have waited until half time to make the change and protect the youngster from the embarrassment of being substituted during the first half, given that he was already 'playing without confidence'?
McNair's error-strewn performance shouldn't be allowed to take away from the progress he has made in such a short time.
In half a dozen appearances since his debut against West Ham in September, McNair has hardly put a foot wrong.
He has coped superbly with the pressure that comes with playing for Manchester United and has also shown a maturity beyond his 19 years. Every player can be forgiven for having a bad game and look at the bigger picture - United won the game.
That shouldn't be a surprise though. Van Gaal's team has won every game that McNair has started, with the only time he has suffered defeat being when he appeared as a substitute in the Manchester derby.