The 23-year-old moved to Stamford Bridge in August and is the latest big money buy for Chelsea as they continue their Champions League quest.
They were inches away from success in the 2008 final when John Terry's penalty went inches past the post and since then the competition has started to be described as an obsession for the club.
Mata faces a Valencia side he knows as well as anyone after spending the last four years at the club.
"It is going to be a special game for me and while I had a great time there my priority is now Chelsea and I will go out to do my best," said Mata.
Chelsea manager Andres Villas Boas does not believe his team will be tired following two consecutive games which has ended with them down to ten men.
"It should be okay and nothing to worry about as it is still early days in the league," he said, while admitting his team selection last Saturday against Swansea, which included leaving Frank Lampard on the bench, was influenced by this.
"We sent nine fresh players onto the pitch that didn't play Wednesday and I think that was important."
After one game so far in Group E, Chelsea lead the way following a victory over Bayer Leverkusen while Valencia had a disappointing draw away to Genk.
The return of Mata has hit the headlines in Valencia and his former room-mate, Pablo Hernandez is also looking forward to the game.
"On the pitch you don't have friends and so each player fights for their own team and we will be looking to beat Chelsea," said Hernandez.
"I still talk a lot to Juan and it is going to be a good to play against him in a competition like the Champions League."
Valencia have started well in la liga and held Barcelona to a 2-2 draw last week but lost 1-0 away to Sevilla at the weekend.
"The game against Barcelona is important as a reference and shows that if we play that well then we can beat anyone," said Hernandez.
"It is true that Chelsea play a different style to Barcelona with the ball but it will still be a demanding game.
"We need to win and pick up three points especially after the Genk match but it will not be easy against Chelsea who are one of the best teams in Europe."
Source: AFP
Source: AFP