Arsenal were quick out of the blocks in the second half as Smith Rowe equalised, showing confidence and skill to dodge two tackles and curl home just two days before his 18th birthday.
The 29-year-old appeared to struggle to adapt to Mourinho's tactics and he was eventually moved on to Arsenal, where he seems to have found a football philosophy more in line with his own style of play.
Emery was delighted for the Croydon-born midfielder, who joins a number of promising English players on the fringes of Premier League teams.
"It's more for the player that he has the feeling that his situation is resolved and he can purely concentrate on the game". "Arsenal has always played offensive football but under Jose, the team at United was playing a bit differently".
'Then they are winning every day in training sessions to be with us. "What's important for us is that every young player is working very hard". "He started today because his performance is good each day in training".
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The highly entertaining game ended in a 1-1 draw after 90 minutes.
Simeone was also happy with his youthful side, who were deputising for senior players like Antoine Griezmann, Diego Costa and Koke. If they work hard, prepare well, they can take these opportunities.
Against the run of play, Atletico took the lead with their first shot on target as Luciano Vietto directed a fine header into the far corner on 41 minutes.