Genoa scored to briefly threaten a shock before goals from Martin Baturina, an Assane Diao brace and Nico Paz gave Como a comfortable away win, with debutants Moise Kean and Samuele Ricci both involved as Fàbregas's side moved top of the table.
De Rossi was left to reflect on a "heavy defeat" against opponents he rates the strongest he has faced since taking charge of Genoa.
"They're not just technical, they're physical too — it's the team that has impressed me most since I started managing Genoa. They're strong," he said. "Against Como you have to be perfect to think about taking points home."
The Genoa head coach felt his side's positive start deserved more, lamenting a lack of precision in front of goal.
"The attitude, the initial approach to the game, was good. We looked for our game plan, attacking in behind. We scored, and with a bit more precision we could have had more shots on target," he said, adding he wanted to review the match before passing final judgement.
"I'd rather watch the game back before giving my verdict."
He praised his supporters regardless of the scoreline: "The crowd is incredible. I told the lads to try switching off their phones and not read social media — we play inside the stadium, and the answers come from the fans inside the stadium." He also gave an update on debutant Kingsley Ehizibue:
"I'm pleased with how he came on. He hasn't trained with us — I could only give him some basic information. He has the physique for it, and it's a role we've been searching for all summer."
Fàbregas, by contrast, was focused less on the scoreline than on his side's approach throughout.
"We don't just prepare for matches, we try to prepare players with the right mentality. Even in training we do this, sometimes starting a drill with a team down a man or 0-2 down," he said. "If you fall asleep in Serie A, it can hurt you. We gave Genoa two chances, one badly defended and one well defended.
"I liked the concentration and desire. Whether we win 4-0 or lose 4-0, it's about having the right mentality on the pitch."
He singled out Diao's display as his best in a Como shirt, while cautioning against excessive expectations of his younger players: "He's 20 — we can't expect players that age to play well every time. They need educating."
New arrival Samuele Ricci said the result left a strong first impression after his loan move from Milan.
"Really positive feelings from the very start. They welcomed me with great warmth," Ricci said. "It's still early to say, but I feel good with these lads."