Milan defeated Roma twice in Serie A this season, but both wins came against sides managed by Jose Mourinho. The 3-1 loss in January led to Mourinho's dismissal and with De Rossi at the helm Roma have lost only once in 11 league games.
"They are in good form and are coming off the back of the derby win, and they have only lost with Inter Milan," Pioli told a press conference ahead of Thursday's first leg.
"Since Mourinho left they have changed, so we haven't rewatched their games against us in the league, but rather those with De Rossi on the bench."
While Pioli knew what to expect of Mourinho's Roma, but is unsure what style of play his side will face at the San Siro.
"I think De Rossi is doing a great job, but I'm not talking about their play, but about knowing how to be credible to get the team to give him total availability," he said.
"When Mourinho was there it wasn't easy to overcome Roma who defended with so many players, it won't be easy tomorrow either, not knowing what choices De Rossi will make.
"A lot of teams usually play one way, then they change when they face Milan because they are aware of our attacking players."
Real Madrid and Manchester City served up a six-goal thriller on Tuesday in the Champions League, but Pioli believes his side are not far off that standard.
"We reached the semi-finals last year, which means we were there. I don't think we're very far apart," Pioli said.
"The Europa League is below the Champions League, but there are great teams. No one would be surprised if ourselves, Liverpool or Bayer Leverkusen were playing in the Champions League."
