Atletico have been linked with the Argentine for a while and they will be delighted to have finally got their man, especially with their Portuguese midfielder Tiago Mendes out with a broken leg. Fernandez has played under Simeone before, winning the Argentine Clasura in 2008 with River Plate so the manager will be well aware of what to expect from the tough tackling midfielder.
Fernandez has played a key role in Celta de Vigo's good start to the season. While the likes of Nolito, Iago Aspas and Fabian Orellana have been grabbing the headlines for their goals, Fernandez has been working hard in midfield and he will be a huge loss to Eduardo Berizzo's team.
Atletico hoping for a title charge
Atletico have been enjoying a good season so far and are currently joint top of La Liga with Barcelona. They also came through their Champions League group and will face Dutch champions PSV Eindhoven in the last 16. Simeone has been building a good squad and Fernandez will be another good addition as they look to win silverware this season.
After Fernandez was revealed as an Atletico player the clubs sporting director, Jose Luis Perez Caminero described him as a "player of proven experience" and someone with "great tactical and technical qualities" who could help the club in their pursuit of glory.
Kranevitter available to play
Fernandez isn't the only Argentine midfielder to become eligible for Atletico Madrid today with another former River Plate star, Matias Kranevitter now officially an Atletico player. Kranevitter agreed to join the club in the summer but he wanted to stay with the Copa Libertadores winners until after the FIFA Club World Cup which was held in Japan just before Christmas. River qualified for the final but lost to Atletico's main league rivals Barcelona.
With Fernandez and Kranevitter joining an already talented squad Atletico will be hopeful that they can challenge on all fronts this season and with Simeone proving himself as one of the best coaches in world football it could be another successful year for the club.