Manager Andre Villas-Boas paid around 30 million euros to lure the playmaker to the Premier League and the 23-year-old made an immediate impact, scoring in an opening 2-0 home Group E win over Bayer Leverkusen.
"I'm looking forward to seeing him and playing against him, we all are," Valencia winger and Mata's former room-mate Pablo Hernandez told reporters.
"It will be special for him as he returns to what was his home for a number of years but there are no friends out on the pitch. We'll be giving our all to beat them and Juan, then afterwards I can wish him all the best."
"He is a player of magnificent talent. He has been magnificent in the Champions League for Valencia and is showing his potential in the Premier League now as well," said the Chelsea boss.
Although Spain striker Fernando Torres's 39th minute sending-off was a blot on the afternoon, he looked back to his old sharpness and scored for the second successive league match.
His stoppage-time goal was another bonus for Villas-Boas who has a strong squad at his disposal for a match he describes as Chelsea's toughest in the group.
"It is going to be a very difficult game because you saw what Valencia can do against Barcelona," he said. "But if we get a win there it will be a major step towards qualification."
