Sources have told ESPN that Felix signed a new contract with Atletico until 2027 ahead of the loan move before accepting a move to Chelsea, with both clubs in advanced talks over a deal for the Portugal forward to join the Premier League side on a six-month loan.
Sources added the initial fee is in the region of €11 million, although an option for a permanent move will not be included in the deal, and Chelsea will pay cover Felix's wage in full.
Atletico chief executive Miguel Angel Gil Marin suggested last month that Felix's relationship with head coach Diego Simeone had worsened and that the club were open to agreeing a move away.
"For reasons it isn't worth getting into -- the relationship between him and the boss [Simeone], the minutes played, his motivation right now -- it makes you think that the reasonable thing is that if there's an option that's good for the player, good for the club, we can look at it," Gil Marin said.
He started in Atletico's 1-0 defeat to Barcelona in Sunday's league game and has scored five goals in 20 appearances this season.
The forward became Atletico's club-record signing back in 2019 when he arrived in a €126m deal from Benfica.