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.