The veteran missed seven months of action after damaging his Achilles tendon in a 1-0 defeat to Palermo in April, sparking rumours he could call time on his distinguished career.
But the 40-year-old rubbished that notion and returned to the bench for Inter's 3-0 triumph over Udinese on Sunday.
Mazzarri may now give Zanetti some playing time in the showdown with Livorno on Saturday, and believes the Argentinian's return will be a huge boost.
"Zanetti will be on the bench, we'll see how the game goes and decide what to do," he said.
"I look at how the players perform on the pitch, not their age. Zanetti is world class, when he's fit he'll be a huge help to us."
With 27 goals to their name in Serie A this season, Inter possess the best attacking record in the league.
But Mazzarri has warned his side to improve defensively, and urged the team not to take the challenge of Livorno lightly.
"Inter have the best attack? That's good," he added. "We try to play attacking football and score lots but we must tighten up at the back.
"If you want to achieve good results, in line with the size of this club, the team needs its own identity.
"We must play every match as if it were a final. That's what I try to drum into the players. A team that wants to be challenging for the top spots cannot afford to underestimate a single game."
Mazzarri - whose side sit fourth in the table - previously managed Livorno, leading them to promotion from Serie B in 2004.
And while he has good memories of his time at the club he insists his focus is purely on Inter.
"I have close ties with Livorno," the 52-year-old continued. "There's mutual respect between me and the president, but tomorrow I want what's best for Inter."
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