France international Mathieu, 30, has only called Messi and Neymar a team-mate for five league appearances after switching from La Liga rivals Valencia.
Centre-back Mathieu said Neymar (six goals) and Messi (five) were clicking better on the field than in Barca's 2013-14 campaign.
"This year [season] so far, the duo is working quite well if we compare to last season," Mathieu said.
"Neymar needed time to acclimate himself to the way of play of Barcelona.
"We are happy having them within the squad.
"In my opinion it's one of the best duos in the world."
Returning to his homeland for a UEFA Champions League clash with PSG in Paris on Tuesday, Mathieu said the match was significant to him as a former rival of the now-powerhouses when he played with Sochaux and Toulouse.
"Coming back to France is a great thing, that's different. It remains a football game. Being in Barcelona is a great thing for me," he said.
"Coming back to the Parc des Princes will be an important game for me, but I don't get under pressure."
The absence of striker Zlatan Ibrahimovic (heel) will be a noticeable one for the Ligue 1 champions, although Mathieu claimed he wanted to match up on the Swedish star.
"PSG have great players but we know they play different without Zlatan," he said.
"They change their style a lot without him. His absence will be better for us possibly and worse for them.
"Personally I wanted to play against him but tomorrow it will be [Edinson] Cavani playing probably. He's also a strong player but different style."
