Gabriel Heinze arrived at Manchester United on Tuesday and instantly pledged to show he was dedicated to the club.
The Argentina defender is yet to play for his new club after performing for his country at the Athens Olympics.
"Some say I did the right thing, others say I didn't. But I'm never going to say 'no' to Argentina," Heinze told the club's official website.
He added: "I'm going to show I can shine in the Premiership. I can't wait to play for Manchester United."
Heinze, who cost £6.9m from Paris St Germain in June. missed United's two-legged Champions League third qualifying round tie and four Premier League matches.
He helped Argentina reach the Copa America final in Peru in July and win the soccer gold medal at the Athens Games last month as one of their three permitted over-age players.
The 26-year-old then flew back to Lima for Argentina's World Cup qualifier on Saturday, a 3-1 victory over Peru in which he won his 16th cap.