Alex Ferguson is in no doubt about the identity of the man his players have to mark out of the game to prevent Roma scoring in tomorrow night's Champions League quarter-final. "Stop Totti and we won't blow it in Rome," the Manchester United manager said before his team's first visit to the Stadio Olimpico.

Ferguson insists United cannot afford to allow the Italian club to build a significant lead for next week's second leg at Old Trafford and admitted the Roma captain is the player they must stop to stay on course for a repeat of the 1999 treble. "(Francesco) Totti is the architect of everything they do and we have to decide how best to deal with him," he said. "The issue is what we do with our centre backs and whether we push one on to Totti but we can't let him control the game.

"We are not underestimating our task because it'll be difficult, and it's probably the toughest of all the quarter-finals. It's a game where we'll have to be right tactically because if we're not, we're in trouble."

Roma are second in Serie A and the 1984 European Cup finalists have lost only once in 15 home games this season. They eliminated Lyon in the last 16 and Ferguson is fully aware of the challenge facing his team. "The tie requires two good performances," he said. "We have a lot of respect for Roma. They beat Lyon, who I regarded as one of the favourites, in the last round and didn't lose a goal."

United have problems in defence with Nemanja Vidic and Gary Neville injured, so Wes Brown is set to make his first Champions League start since the defeat at FC Copenhagen in November. Vidic broke his collarbone when he fell in the first half of United's 4-1 win over Blackburn on Saturday and is likely to be out for four to five weeks.

Ferguson is set to leave Louis Saha behind in Manchester. The French forward has been absent for five weeks with a hamstring injury but is now nearing full fitness. Ferguson believes Saha is okay to fulfil bench duties in Rome's Olympic Stadium. However, with so many key games looming, Ferguson is tempted to leave Saha behind to gain extra match practice in Thursday's reserve team encounter with Blackburn at Christie Park.

Roma players Christian Panucci and Mancini have warned Cristiano Ronaldo he cannot expect "protection" from the referee. Panucci said: "Ronaldo's class is unquestionable. But I believe he abuses his dribbling talents and exaggerates his falls. A lot will depend on the referee but in Roma Ronaldo will not be daring enough to do it because we will not fall for his tricks."

Mancini added: "I've watched Premier League matches and he is protected by referees. I don't think he will be on Wednesday."

United have sent warnings to supporters travelling to Rome to help them avoid potential trouble spots in the capital. "We have advised them to avoid one particular bridge and one piazza because they tend to be congregating places for Roma fans and there have been a few knife incidents," said Phil Townsend, United's director of communications.

Meanwhile, Chelsea's Joe Cole has his eyes on a comeback in the Champions League match against Valencia tomorrow night. Manager Jose Mourinho said earlier this season he feared the midfielder would miss the remainder of the campaign with a stress fracture of the foot.

But Cole said yesterday he is "close to being absolutely perfect" in terms of fitness.

Guardian Service