La Liga: Valencia v Barcelona
The France international departed Mestalla during in July after five seasons at Valencia, during which he became an integral part of the team's defensive unit as he switched from left-back to a central berth.
Barcelona signed the 31-year-old for €20 million and he has settled well, offering an experienced option in a couple of positions at the back for coach Luis Enrique.
Barca go into Sunday's clash on the back of two fine, Lionel Messi-inspired wins, beating Sevilla 5-1 in La Liga and then defeating APOEL 4-0 in the UEFA Champions League.
The Argentina striker scored a hat-trick in both matches, becoming La Liga's all-time leading scorer against Sevilla and then the Champions League's equivalent three days later.
And Mathieu is adamant that, although Valencia have started the season excellently, his ties to the club will not prevent him from going all out for the win.
"It's a special game for me but I won't play any differently," he told Barcelona's official website. "It's always nice to play at Mestalla, I was there for many years.
"I don't know how they will receive me – I believe I did myself justice there. [But] now I am 100 per cent focused on Barca.
"We have to pick up three points to keep up there against a rival who are right behind us. We are going to Valencia to take all three points."
Barcelona can return to the top of the Liga table if they win and Real Madrid suffer an unlikely loss at Malaga, but they will have to contend with fourth-placed Valencia's impressive home form.
Nuno Espirito Santo's men dropped their first points at home this season after Athletic Bilbao earned a 0-0 draw in Valencia's last Mestalla appearance earlier this month.
Valencia's eye-catching start to the season has been put down to a significant injection of cash from new billionaire owner Peter Lim, and close-season arrival Andre Gomes believes it is against teams such as Barcelona that the club can prove their credentials.
"They [Barcelona] are a great team and we want to get back to being a top team," he told Valencia's official website.
"It's in these games that we want to show that we are doing well. We are playing at home and will do everything possible to win the game."
Espirito Santo should be able to call upon the services of striker Paco Alcacer once again after he recovered from a hamstring strain, while Dani Parejo could start after getting over a similar problem.
Meanwhile, Barcelona have almost a clean bill of health after Sergi Roberto received the all-clear following his groin injury, but Thomas Vermaelen looks set for five months out as he prepares for hamstring surgery.
Messi scored a hat-trick in a 3-2 triumph for Barcelona in this fixture last season, but Valencia exacted revenge at Camp Nou in February of this year - also winning 3-2.