The 19-year-old completed a switch to Wanda Metropolitano for £113 million ($139m) at the start of July, becoming the most expensive signing in Atletico's history.
Felix had attracted attention from a number of Europe's biggest clubs, including Real Madrid, Bayern Munich and Manchester United, but he opted to continue his development with Atletico.
The 19-year-old's vast potential has caught Diego Forlan's eye, with the ex-Uruguay international now backing him to achieve great success in the Spanish capital.
"I saw him at Benfica and he did spectacular things," Forlan told Marca of Joao Felix.
"He's a different player and he's lucky to now be part of a hard-working and humble team.
"They know that they've achieved what they have through effort and sacrifice and some of that is down to the coaching staff.
"If he's humble, respectful and can listen, he'll be a player who grows up to give us a lot to talk about. He has a big future."
Forlan played for the Rojiblancos for four years, scoring 74 goals in 134 matches while picking up a Europa League winners' medal in 2010.
The 40-year-old has been impressed with the work that Diego Simeone has carried out since taking the helm at Wanda Metropolitano in 2011.
Atletico have also reached two Champions League finals under Simeone only to lose both games to cross-city rivals Real Madrid, in extra time in 2014 and on penalties two years later.
The Uruguayan believes that Atletico Madrid are on the right path to finally win the European Cup one day and thinks that they can now be considered one of Europe's most feared clubs.
"They came very close twice," Forlan said.
"Once leading until the 93rd minute and losing the other on penalties.
"They're on the right track but everyone prepares themselves to win the Champions League.
"Nobody wants to cross Atletico because of what they've achieved. They've earned respect."
Forlan went on to praise the club's steady progress over the last decade before singling out Simeone for his influence in the dugout, adding: "They grow year after year.
"They changed the stadium and they've shown their development in the most important way - with results.
"Simeone's arrival has meant that year after year the players change but the requirements are the same."