Cape Town - AS Roma sporting director Monchi believes Atletico Madrid, rather than Chelsea, are their toughest opponents in Group C.
The Italian side kick off their Champions League campaign on Tuesday against Atleti, while the Blues face the group minnows, Azerbaijani champions Qarabag.
"Atletico Madrid are the most complicated opponents in the group," Monchi told Movistar Plus.
"They’re a team that knows the tournament perfectly well and in my view are tougher to face than Chelsea.
"They’ve been working together for three to five years, with the same coach (Diego Simeone) and rarely changing players."
Although undefeated after three games in La Liga, Atleti have been held to two draws already.
Roma, meanwhile, won their Serie A opener against Atalanta but were recently beaten 3-1 by Inter Milan.
TUESDAY, SEPTEMBER 12
Bayern Munich v Anderlecht - 20:45
Barcelona v Juventus - 20:45
Celtic v PSG - 20:45
Benfica v CSKA Moscow - 20:45
Manchester United v Basel - 20:45
Olympiakos Piraeus v Sporting CP - 20:45
Roma v Atletico Madrid - 20:45
Chelsea v Qarabag - 20:45
WEDNESDAY, SEPTEMBER 13
Liverpool v Sevilla - 20:45
Porto v Besiktas - 20:45
Real Madrid v APOEL - 20:45
Maribor v Spartak Moscow - 20:45
Shakhtar Donetsk v Napoli - 20:45
RB Leipzig v Monaco - 20:45
Feyenoord v Manchester City - 20:45
Tottenham Hotspur v Borussia Dortmund - 20:45