Carlo Ancelotti said playing Napoli away will be Real Madrid's toughest test in the Champions League group stage this season, ahead of the 14-time European Cup winners' visit to the Diego Armando Maradona Stadium on Tuesday.
Madrid and Napoli both have three points ahead of matchday two, after Madrid beat Union Berlin 1-0 at the Santiago Bernabeu and Napoli won 2-1 away at Braga.
Ancelotti coached Napoli for a turbulent 19 months between 2018 and 2019, before joining Everton and then returning to Real Madrid, where he has won six trophies during his second spell in charge.
"Napoli are one of the best teams in Italy," Ancelotti said in a news conference on Monday, when asked to assess last season's Serie A winners. "They've maintained the same structure [under new coach Rudi Garcia]. They have a high level, they have individuals, and an atmosphere which pushes them.
"It's a return to the past. I had positive moments there, in a marvelous city. Some good moments, some less good. It's the most difficult game in the group."
Ancelotti refused to be drawn on his relationship with Napoli owner Aurelio De Laurentiis, who has previously said that the club's fans never took to the coach during his time there.
"I have a very good relationship with everyone," Ancelotti said. "I've always said that when there's no 'feeling' between a club and a coach it's best to leave. It was the right decision for Napoli and for me too. Now I'm back with the best club in the world."
Jude Bellingham, scored Madrid's winner against Union Berlin and already has seven goals for the club in all competitions, has drawn praise from all directions this season, most notably from his teammates who train with him every day.
"World class players, players who make their mark on a team, have the characteristics that Jude has," midfielder Federico Valverde said. "Real Madrid is another step up and he's dealt with it incredibly well.
"He's a good teammate, he's got a good head on his shoulders. For sure he could be a captain who defines an era at Real Madrid."
Bellingham's playing time, however, has come at the expense of Madrid veteran Luka Modric, who was an unused substitute in the team's last two games.
"It's surprising that Modric isn't having the same role as he's had in the past," Ancelotti admitted. "He played the first Champions League game, he played the [Madrid] derby. It's a decision.
"One day it's Modric, another day it could be [Toni] Kroos, it could be [Aurelien] Tchouameni, [Eduardo] Camavinga. In every news conference there's a question like this and there isn't much to explain. We have seven midfielders and we play with four."