Much of the match was played in an eerily quiet stadium with the Curva Nord barely raising a murmur.
“There are lots of factors that come into play when you evaluate a performance,” Mazzarri told reporters. “The lads we have here aren’t used to playing with this sort of pressure. They need support at home and at times it was like we were playing behind closed doors
“If you’ve played the game you know the sort of situation I’m talking about. When we put Catania under the cosh, the crowd came alive: that’s what we need all the time.”
Inter pulled out of a deal to swap midfielder Fredy Guarin for Juventus striker Mirko Vucinic following protests by fans outside the club’s offices.
The Colombian was not included in the squad for Sunday’s match and his Inter future is uncertain.
The fan group Curva Nord also issued a statement criticising new club president Erick Thohir, saying they wanted to hear less business talk and telling him to get more involved in the running of the club.
Thohir, who made a rare San Siro appearance on Sunday, has watched most games on television from Indonesia since taking over from Massimo Moratti in November.
Fans have been angry at the lack of activity in the transfer market and Thohir’s warning that the team is in a transition phase.
“The players are under a lot of pressure,” said Mazzarri. “It’s not about their age, I’m talking about players who are at a club where results are expected.
“Lots of them weren’t used to trying to reach the top or performing with the pressure they find themselves under here, I’m talking about the Jonathan’s, the Alvarez’s, the Kuzmanovic’s,” he said, referring to three of Inter’s less experienced players.
Thohir spoke briefly to Italian media when he arrived in Milan for the game.
“I’ve come here to be close to the team and because I have some meetings and also because of the transfer market,” he said.
Mazzarri added: “The club knows what I asked for in the transfer window. I hope they can do those things, but if they don’t then we’ll continue working with this group.”
“This is said to be a transition year, so I ask the Curva and our fans again to give us a hand because we need them.”
(Writing by Brian Homewood, Editing by Ed Osmond)
(Reporting by Brian Homewood)