Barca sit second in the table, two points adrift of leaders Real Madrid, but gained ground with their win at Granada last weekend as Carlo Ancelotti's men were held at home by Villarreal.
Jemez says Barca cannot afford to take their foot off the gas in the title race, but believes Luis Enrique is the man to deliver the Liga trophy to Camp Nou.
"They have recovered an intensity when they lose the ball and I think that he will triumph in Barca, I have no doubt about that," Jemez told Sport. "He has a very good team and he is a very good coach, and that formula almost always works well.
"But it's also true that, in Barcelona, coming second is not good enough. That's the pressure of a big team, for some coming second is great but there it’s not enough.
"But I have a lot of faith in Luis because I think he's a very qualified coach with a lot of knowledge.
"And I think little by little he's getting Barca to play very well."
Barca attacker Rafinha has also noticed an improvement at Camp Nou and hopes to earn himself more game time.
"We've been getting better and better," he said. "There haven't been any big changes. We just feel better with each game we play.
"I've been getting better, getting more confidence, and I'd like to get more minutes. I feel quite comfortable in the team. I'm here to help.
"In order to keep getting minutes on this team you have to constantly be showing what you can do."
Sergio Busquets will miss Sunday's fixture after suffering ankle ligament damage against Villarreal.
A return date for Busquets has not yet been confirmed ahead of crucial games against Manchester City and Real Madrid this month.
Rayo Vallecano are 11th after two wins from their last three matches, but have conceded 34 goals and scored only once in their last eight league clashes with Barcelona.
