The Giallorossi were drawn to face the Spanish side in the last-16 of the competition during Friday's draw in Nyon.
The first leg takes place in Andalucia on March 12, with the second leg coming at the Stadio Olimpico a week later.
“Sevilla are one of the best sides left in the competition," Fonseca said.
"They are a side we know well, with a coach we know well, and good, experienced players. They are fourth in the league but level on points with Atletico Madrid. They are a good side."
He added: “I think Sevilla have changed a lot over the last year or so, as we have, and they have new players and a new coach. They have invested in a lot of players and have changed the way they play. They have their approach, and we have ours."
Asked if it was an advantage to have the second leg at home, Fonseca pointed out that performing in the first leg would have a huge impact on that.
"We’ll see. The first game is always really important," he said.
"So we are playing away, and it’s always important to get a good result before you come home. Sevilla have that home advantage first, but if we put in a good display there then coming home could be an advantage for us."
Fonseca's comments were echoed by the club's Chief Global Sporting Officer Manolo Zubiria, who represented Roma at the draw in Nyon.
"At this stage every side left is strong," Zubiria said.
"If we want to go far in this competition we have to be ready to face anyone. Sevilla have great history in this competition but we are ready for the challenge.”
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