The 31-year-old's contract at his previous club Sevilla was terminated just four months after he joined the side in December, with then coach Jorge Sampaoli saying the former Real Madrid player failed to live up to their expectations.
After a difficult start to the season, Diego Simeone's Atletico have hit their stride and prior to the international break, they eased to a 3-0 home victory over Valencia to extend their unbeaten run in LaLiga to 10 games.
Barca, in second, are eight points clear of Betis, in fifth, and if they can extend that gap to 11 with three games left to play, their spot in the Champions League will be secure.
Messi came off the bench to score twice and lead Barcelona to a 5-2 home victory over Real Betis on Saturday.
Pellegrini returns to the Spanish League for the first time since 2013 when he was in charge of Malaga.
LaLiga returns on 11 June with El Gran Derbi between Sevilla FC and Real Betis.
La Liga president Javier Tebas said Sunday he hopes the resumed season will start on June 11 with the Seville derby, "one game for all of Spain".
Lionel Messi produced a hat-trick of assists as Barcelona pulled off a gutsy 3-2 win at Real Betis after trailing twice in an enthralling match in La Liga on Sunday, finally giving the Catalans something to celebrate after a week of upheaval.
Amr, who represented Wadi Degla, Enppi and El Gouna in Egypt, had a massive impact on his team since he joined the club from Orlando City SC.
Real Betis veteran, Joaquin Sanchez, believes that the arrival of Nabil Fekir to the club will have a huge impact on the team.
Setien is to leave the club by mutual consent, the club said on Sunday after he enjoyed a memorable end to his two-year tenure by winning 2-0 at Real Madrid.
Madrid’s harrowing season came to a fittingly miserable end on Sunday as they slumped to a staggering 12th La Liga defeat, losing 2-0 at home to Real Betis.
Messi highlighted his magic night with a brilliant goal that led Real Betis fans to applaud the performance of the Argentinean.
Nigeria international already looking key to the Yellow Submarine’s chances of success this season.
Benzema sustained the problem in Sunday's 2-1 La Liga win at Real Betis and was taken off at halftime.
Injury-hit Real Madrid snatched a 2-1 victory at Real Betis on Sunday, earning their first La Liga win of the year.
Real Madrid have made a gloomy start to the new year but as the floundering European champions prepare for a difficult trip to Real Betis on Sunday, hope has arrived in the unlikely figure of teenager Vinicius Jr.
Ruiz is Napoli's second signing of the close season after striker Simone Verdi joined from Bologna.
Vermaelen joined Barcelona in 2014 from Arsenal but only found success with the club this season.
It is easy to forget how talented Messi is when he performs at such a consistently high level that you simply expect him to work wonders in every match.