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."
Villas-Boas is unconcerned that the emotion of the day will get to Mata.
"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.
Valencia started their European campaign with a disappointing 0-0 draw at Racing Genk.
They were in fine form in La Liga until Saturday when they lost 1-0 at Sevilla.
Just three days before that they had European champions Barcelona on the rack at the Mestalla, twice taking the lead and wasting chances to seal the win before being pegged back to a 2-2 draw.
TORRES SHARP
Chelsea are in high spirits following a 4-1 win over Swansea City on Saturday which left them third in the Premier League, three points behind Manchester United and Manchester City.
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.
Chelsea were also boosted by the return of Ivory Coast forward Didier Drogba as a late substitute after a four-week injury lay-off.
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."
Probable teams:
Valencia: 1-Diego Alves; 23-Miguel, 4-Adil Rami, 18-Victor Ruiz, 22-Jeremy Mathieu; 19-Pablo Hernandez, 5-Mehmet Topal, 10-Ever Banega, 21-Daniel Parejo; 9-Roberto Soldado, 14-Pablo Piatti
Chelsea: 1-Petr Cech; 2-Branislav Ivanovic, 4-David Luiz, 26-John Terry, 3-Ashley Cole; 12-John Obi Mikel, 16-Raul Meireles, 8-Frank Lampard, 10-Juan Mata; 9-Fernando Torres, 11-Didier Drogba.
