Barca host the Serie A side after suffering a surprise reverse in last month's last 16, first leg at the San Siro and are reeling after being dumped out of the King's Cup and beaten in La Liga by arch rivals Real Madrid last week.
They are still 11 points clear at the top of Spain's domestic league but their hopes of a third European crown in five years are hanging by a thread.
"The desire this club has to win the Champions League is enormous and we will all perform at 100 percent," Fabregas said.
"But it's not necessary to start the match at full throttle," added the former Arsenal captain.
"Over-excitement is not good. We have to be intelligent as it's not just about playing with the heart. The head will be important too."
Fabregas, one of 11 Spain internationals in the Barca squad, noted that the club have a host of players who have proved they can perform on the biggest occasions.
"We are very capable of playing at this level, we have played finals in the Champions League, the European Championship and the World Cup," he said.
"This team has always performed at key moments. It's the motivation, the desire, the confidence to say if we play well and win it will be an incredible boost in morale."
Before Milan's visit, Barca host La Liga strugglers Deportivo La Coruna at the Nou Camp on Saturday (1900 GMT).
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- La Liga
- Champions League