The ex-Valencia star has pinpointed the Argentine sensation as the biggest threat to his side, while declaring that the Stamford Bridge side can reach the Champions League final
The Argentine is the leading scorer in the competition, with his 14 goals so far already a record.
"If he is having a good day then watch out," Mata told Marca. "Barca have other important players, such as Xavi and Andres Iniesta, but it is clear that Messi is the main danger and the one who can unbalance a match the most.
"It is clear that Messi is the main danger - we will try to make sure he does not have a good day"
The Spaniard refuses to write off Chelsea's chances, though, claiming that they could spring a surprise as long as they keep a clean sheet on Wednesday.
"It is crucial that we get a good result at Stamford Bridge and that we do not concede any goals," Mata added.
"If those conditions are met, we can reach the final because we can score a goal at Camp Nou.
"But it will be hard because Barca create several chances to score in every match.
"They are a tough side to beat, especially in a two-legged tie, but if we have a good day, it is possible."
The attacker also insisted that countryman Fernando Torres still plays a vital role in the team despite having only scored seven goals to date this term.
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"He grabs assists, pressures opponents and leaves you without markers. I think he is having a good season and he will soon score again."
Asked about this year's managerial changes at Chelsea, the former Valencia man stated that Roberto Di Matteo's appointment has turned the club's fortunes around.
"With Andre Villas-Boas, we could not win, no matter what we did," he noted.
"With Di Matteo, we have regained our trust and that is seen on the pitch."
Goal.com will be running a dedicated LIVE! on Tuesday and Wednesday from 16.00-00.00BST to keep you up to date with all the latest about the Champions League semi-finals, while also offering you the chance to discuss with our team of experts how a combined Bayern/Madrid XI could look like, and what Chelsea must do to shut out Lionel Messi and stop Barcelona.