On 4 January 2012, the club received a US$1.5 million bid of Argentine Primera División side Newell's Old Boys for Díaz, but the same player rejected the deal, to play the Copa Libertadores, being also retained with pay rise of part of directive. He was candidate for be the club's captain, with the departures of José Rojas and Marcos González to the Brazilian Série A, but finally with the return of Rojas to the club because of problems with his new contract, Pepe remained as captain. Díaz started the season of fine form scoring a penalty goal in a 3–1 win over his old club Deportes La Serena in the first game of Universidad de Chile at the Apertura Tournament. On 25 February, he missed a penalty kick against O'Higgins in a 3–0 loss, that was saved by the adversary's keeper Luis Marín for the fifth week of the Apertura. On 10 March, he played a great match giving two notable assists in a 6–0 thrashing over Audax Italiano, then being called up by Borghi to the national team for play the Copa del Pacífico against Peru at Arica and Tacna. On 4 April, he was named the man of the match in a Libertadores 1–0 victory over Godoy Cruz at Mendoza, making another excellent match for his team on 29 April in the Chilean derby against Colo-Colo, in where he scored the first goal of the game and also made three assists, after a successful free kick in a 5–0 home win at the Estadio Nacional. On 9 May, was confirmed that Díaz was sold to Swiss Super League side Basel for $4 million. On 11 May, he scored a 30-yard goal in a 6–0 rout over Deportivo Quito for the Libertadores. FC Basel After winning the Apertura with Universidad de Chile, Díaz joined the Swiss side Basel in the summer of 2012, on a four-year deal. He joined Basel's first team during their 2012–13 season under head coach Heiko Vogel. Díaz played his club debut on 17 July in the 2–0 away win against Flora Tallinn in the 1st Leg of the second qualifying round to the 2012–13 UEFA Champions League and he received good compliments. He played his domestic league debut for the club in the home game in the St. Jakob-Park four days later, on 21 July 2012, as Basel played a 2–2 draw with Luzern.