The 39-year-old Argentine has been with the Nerazzurri since 1995 and remains an important part of the team, but concedes that he cannot play forever.
"I'm feeling physically well and I still feel useful at Inter," said Zanetti. "I realise that sooner or later I will have to retire and that moment is not far away. I'm not too worried about it, though.
"It was really tough to be benched against Siena because unfortunately we lost and from the outside you suffer.
"But it was perfectly normal to be on the subs bench and I think the coach (Andrea Stramaccioni) knows fully well that I'm at his disposal."
Zanetti added that he will always be committed to Inter even if he does not start matches regularly.
"It was bound to happen sooner or later and there might be many more games I play no part in," he said.
"I realise this is the birth of a new Inter and I'm here at the coach's complete service – he knows he can count on me both on and off the pitch. That's the way forward."
Zanetti has won Serie A five times and the Champions League on one occasion with Inter.