Vialli said: "I don't think Chelsea are the same team as they were a few years ago. They don't have the consistency any more.
Vialli feels manager Luiz Felipe Scolari, who has set his sights on winning all four trophies this season, was outthought by Roma counterpart, Luciano Spalletti.
He added: "Chelsea went to Rome to settle for whatever they could get and, when you do that, you end up losing the game on most occasions. Roma had a great reaction to this, especially after all the problems they have had recently.
"Spalletti changed the system, with [Francesco] Totti behind [Mirko] Vucinic as well as [Matteo] Brighi supporting [Simone] Perrotta on the flanks, and this made life difficult for [Florent] Malouda. Roma showed great football with the ball to feet and it worked."