The 19-year-old signed for the Madrid outfit from Benfica back in July for €126 million (£113 million):
The 2018-19 campaign was his first in senior football, and he played a key role in Benfica winning the Primeira Liga, netting 15 goals and providing seven assists.
Making the step up to La Liga will be challenging for the Portuguese teenager, but his compatriot Mourinho has said he has a responsibility to perform, per the Portuguese Football Association's television channel (h/t Marca):
"He has a challenge due to the extent of his talent and because of the investment the club have made on him. I believe that he can't escape this responsibility. It doesn't matter if he's 18, 20 or 25. What matters is his talent and the responsibility he has."
However, pre-season games can often be misleading.
Diego Simeone's side start their 2019-20 La Liga campaign with a clash against Getafe on Sunday, and the home fans at the Wanda Metropolitano Stadium will be eager to see Felix make an impact.
He faces the responsibility of filling the boots of Antoine Griezmann, who joined Barcelona earlier this summer after five successive seasons as Atleti's top scorer:
Given he is still only a teenager, that is a big responsibility to be put on Felix's shoulders.
But he has shown enough at Benfica and in pre-season that he has the talent to fill the gap left by the Frenchman.