Swansea head to Stamford Bridge for the first leg seeking to reach a major Wembley final for the first time in their 100-year history.
Club-record signing Hernandez, bought from Valencia for £5.55million, is likely to return to the starting line-up in London, along with top scorer Michu and Ashley Williams, after recovering from a thigh problem.
And if Swansea are to give themselves a chance of winning the tie, Hernandez knows just how crucial it will be to stop Chelsea's creative fulcrum.
He said: "Mata is a big friend of mine. He was my room-mate at Valencia for two years. We used to eat lunch and dinner together too. He's a good player.
"I'm happy that I can play against him and Chelsea but he is a player we need to stop.
"I remember he missed the Premier League game against us because he was injured, but he is back now and is an important player for Chelsea.
"He won't be easy to stop but I have confidence in the Swansea defence."