A 2-2 draw with Genoa and Inter Milan's win against Torino saw Napoli's lead at the tip of the Serie A table cut to one point with two games to play.
Speaking with Naples' Il Mattino, Tardelli said: "Matches like this happen: Inter's result was evidently a factor. I still believe, however, that only Napoli can lose this championship.
"Of course, the Azzurri squandered a great opportunity to almost definitively close the deal in terms of the Scudetto, also because it's not like they were facing an unbeatable team or one on par with the league leaders. Inter are just doing their job.
"They've had a new chance and maybe everything will be decided on Sunday. How did I see the Azzurri? Napoli have always achieved results, I won't say negative, but of a lower level than their standards at the Maradona against less strong teams. Despite the draw, I have to say that I saw the team well: of course, they've dropped a little. Then it's possible that they'll start to get a little scared too."
Momentum with Inter
On Inter and their coach Simone Inzaghi, Tardelli said: "Now Inzaghi will play those who have not played many games: he has fresher players who are eager to show they are good. But if Napoli wins there is no one left. If you have a very clear head, attentive and eager you can drive your legs a little better.
"Having their minds on the Champions League is not a disadvantage. On the contrary. The players can recover. And then, as already seen with Torino, the reserves can easily play, who are not reserves but regular bench players. Unfortunately, Napoli does not have that quality in their squad."
On the future of Antonio Conte at Napoli, he added: "If he wins the championship he could leave. If he doesn't win it he would still be fine. But he has given signs of a farewell"