Milan fell to a 1-0 loss at home to Napoli through Matteo Politano's strike early in the second half, the Inter loanee's fourth goal in his past eight league games - as many as he managed in his first 17 appearances of the campaign.

The defeat leaves Pioli's men nine points behind Inter, having itself been at the summit just a month ago.

Milan was again without striker Zlatan Ibrahimovic against Napoli, as well as Mario Mandzukic, while Ante Rebic and Hakan Calhanoglu have also spent time on the sidelines.

That told as the Rossoneri failed to find a way past David Ospina from their five shots on target, though Pioli pointed out fellow heavyweights Inter and Juventus would also struggle if they were without their best players.

"When you are missing that many players, it does make a difference, especially when they are players who also bring experience and leadership," Pioli said.

"We had to play many games without a real centre-forward, so considering all of that, the team deserves praise for all they've been able to do in the circumstances.

"Try to imagine Inter without [Romelu] Lukaku, Lautaro [Martinez] or [Alexis] Sanchez; maybe Juventus without [Cristiano] Ronaldo, [Alvaro] Morata or [Federico] Chiesa.

"We had more than half the season without Zlatan, while Rebic and Calhanoglu were also injured for long periods.

"We showed all week that this is not just a strong team, but a special one. Unfortunately, there wasn't enough tonight.

"We should've either been more courageous or sat back more, but tonight we tended to be somewhere in-between and were caught in no man's land."

Milan has won just two of its past six league matches and is only six points better off than fifth-placed Napoli, wich also have a game in hand.

With the second leg of the Europa League last-16 tie with Manchester United to come on Friday (AEDT), three days before a trip to Fiorentina, Pioli accepts fatigue is now playing a big part.

"It was a very tough and tiring week. I saw a performance that was sufficient, but that's not enough," he said. "We returned from Manchester at 3am on Friday morning. It's obvious the Europa League is more stressful and damaging than the Champions League, especially when we started in September.

"Having said that, we are Milan and would never want to be without that European presence. We weren't expecting to be in second place, but now that we are here, we want to keep hold of it.

"Inter have the momentum - they really started to step on the gas after their Champions League exclusion and it allowed them a full week to prepare for every game.

"We are not looking ahead or over our shoulders; we just stay focused on our path."

Napoli, previously without an away win in four league outings, is unbeaten in its past six league encounters with Milan at San Siro since December 2014.

The home side's misery was compounded late on when, shortly after Theo Hernandez had a strong penalty shout rejected, substitute Ante Rebic was dismissed for dissent.

Despite being closer to fifth place than first, however, midfielder Sandro Tonali insists Milan is still looking up the table rather than down.

"I don't think there is fear, otherwise we'd lose every game," he said. "We are here and must continue to aim high and not give up now."

"This is not the game we prepared - it was not what we intended to do. Napoli closed down all the spaces and we didn't create enough scoring opportunities.

"The ones we did create with Rafa Leao were not converted and then Napoli scored their first real chance."