Arsenal’s teenage left-back Myles Lewis-Skelly needs to understand “the line is really thin” if he is to avoid disciplinary issues during his career, says manager Mikel Arteta.

The 18-year-old has been booked twice and sent off twice since January.

He was also substituted in the first half of Tuesday’s 7-1 Champions League win at PSV Eindhoven after collecting a yellow card, and then going in hard and late on home defender Richard Ledezma shortly afterwards.

“A lesson to learn for him for sure. We will support and help him. The line is really thin in sport and he needs to understand that,” said Arteta.

“It wasn’t the moment to take any risk. He’s very intelligent, he’s extremely demanding of himself. He’s a perfectionist. He’s got a great environment around him – he’s here to learn as well.”

Lewis-Skelly, who made his senior debut during the 2-2 draw with Manchester City in September, has had an eventful start to his Arsenal career.

As well as scoring a brilliant individual goal in the 5-1 thrashing of City in the return fixture in January, he was controversially dismissed when Arsenal played Wolves at Molineux – a decision later overturned after his team-mates battled to a 1-0 win.

He was again shown a red card in February as Arsenal’s title hopes were dented with a 1-0 home defeat by West Ham.

Arteta, who takes his side to Old Trafford to face Manchester United on Sunday (16:30 GMT), suggested Lewis-Skelly needs to weigh up the odds better when going in for tackles in the future.

“You need to dominate the space you are defending,” added Arteta.

“What is the level of exposure that you have? Is the opponent facing you or not facing you? What is the probability to win that ball and when you have to commit to certain duels?

“Then you have to apply defensive principles [which] are very clear. If you don’t do that at this level it’s a big problem.”