The Argentinian has caught the eye in the last seven months, helping his country to World Cup glory and seeing his transfer value sky-rocket.
Here, Planet Sport looks at what strengths Fernandez can bring to Graham Potter's Chelsea.
Fernandez' passing ability will be a welcome addition to Chelsea after they sold midfield operator Jorginho to Arsenal.
Prior to his exit, Fernandez accumulated the joint-second most assists in the Primeira Liga this season - five.
His ability to scan the pitch before receiving the ball on the half-turn is no doubt an attractive feature for Chelsea, who were in need of new options in midfield.
Age is not a problem
At just 22, Fernandez joins the long list of young talent Chelsea's owners have acquired since last summer's takeover.
He joins the west London side on an eight-and-a-half-year deal with no release clause and will hope to establish himself as a mainstay in the team's midfield.
The record-breaking fee the Blues have paid to Benfica reflects the vast potential Fernandez possesses and the room for him to continue to develop his career.
Fernandez will be no stranger to the high-octane football of the Premier League.
Complimented for his high intensity, the South American featured in all seven games for Argentina at the World Cup as they went on to lift the trophy in Qatar.
The midfielder has picked up seven yellow cards across all competitions for Benfica, tenacity which needs to be channelled but highlighting his resilience in such a crucial position.
Chelsea's main man
Graham Potter has struggled to pick a consistent 11 with his midfield often changing from game to game.
The signing of Fernandez gives Chelsea a lynchpin for the midfield to be built around.
His presence may be the key to the team finding consistent form and identifying a clear philosophy at the midway point of the season.
Fernandez often finds himself in pockets of space where he can use his speed to attack when someone has covered him defensively.
His confidence is certainly reflected in his willingness to shoot.
Fernandez developed a reputation as a goalscoring midfielder in his homeland with River Plate last season and has carried that on with Benfica in the current campaign.