Swansea head to Stamford Bridge for tonight's 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 €7m, 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."
But winger Hernandez, 27, hopes Swansea's good record against the Premier League's big guns will stand them in good stead for the task ahead.
"It's an important game for the club. To play at Wembley in a final would be great," he said.
"We have two matches against Chelsea and it will be difficult.
"Chelsea are a good team and the last winners of the Champions League, but I think we can beat them in Swansea.
"If we score in the first match then it is possible to win in the second one."
Chelsea are the favourites to lift the trophy, with Aston Villa and Bradford contesting the other semi-final.
Failure to do so would heap the pressure on interim manager Rafael Benitez, who was an unpopular appointment among Chelsea fans.
And Swansea manager Michael Laudrup has told the Spaniard he must win silverware to get the fans on his side.
"He (Benitez) has so much experience," said the Dane. "I think he tells his players that what happens in the stands, happens. They must concentrate on playing. You can't change the mind of some people.
"But I still think that with results and more results, suddenly you get more and more will respect and less will be shouting bad things about the manager.
"If everybody in football says you are the favourite you have to accept that. "That is part of being a member of a big club."