The Argentina midfielder played 85 minutes before being replaced in victory over Brentford on Saturday, Chelsea's first Premier League win since December 13.
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Rosenior said, "So impressed. I knew what an outstanding player he was and I was actually at the World Cup final when he played.
"But it's not until you see a player up close and you get to know him and what he's about, and I have to say he's a winner. The reason he always plays is that he trains 100 per cent.
"I've enjoyed working with him and it's his birthday, so I told him to go to a steak house and enjoy some Argentinean steak. He is a magnificent player. More importantly, he's a fantastic character and I love having him in this team."