The Russia international has revealed his disappointment to see the two midfield stars leave the club.
But he admits that the signing of Mikel Arteta and Yossi Benayoun have gone some way to helping them over their loss.
"At the moment, losing Fabregas and Nasri is a catastrophe," the forward told Russian newspaper Sport.
"Watching the Udinese match I said to [Nicklas] Bendtner that now we have no-one so comfortable with passing the ball to Robin van Persie or who ideally utilises the qualities of the other forwards.
"He smiled and replied, 'Yes, the only player who could properly replace Cesc is probably Xavi'.' I'm very glad that I was able to play in the same team as Fabregas - he is a truly class footballer."
But the 30-year-old was pleased with other business done by manager Arsene Wenger.
"When people say no-one is irreplaceable I don't agree. With [Mikel] Arteta or whoever replaces those who have left, we will play different football, not as we played with Cesc and Samir," he said.
"Arteta and Benayoun have long proved themselves in the Premier League, they're not bad signings. And we needed strengthening in those positions.
"Perhaps another holding midfielder wouldn't hurt and, as far as I know, we were trying till the last minute to get [Rennes midfielder Yann] M'Vila, but unfortunately we weren't able to buy him," he added.
"Judging by the quantity and quality of our signings, Arsenal's football will soon undergo some changes."