The Colombian international has only found the net once for the Red Devils, but his fellow South American insists that the striker's overall game has developed.
"Falcao is a better striker now than he was at Atletico for sure. He was injured and changed clubs so he wants more. He is not resting right now. He is a tiger and wants to score more," Luis told the London Evening Standard.
"I enjoyed playing alongside him at Atletico for a lot of games. I remember him in the Super Cup, he scored a hat-trick in about 30 minutes [against Chelsea] but he did a lot of good performances in Atletico and had a lot of good seasons there.
"He is a brilliant striker. He fights for every ball and is probably the best in the world with his head. I know it's going to be difficult but not just against Falcao - United have a lot of good players."
United take on Chelsea in the Premier League at Old Trafford tomorrow.