The combative midfielder has insisted that his side cannot afford to take Tuesday's encounter lightly if they are to progress
The Rossoneri recorded a 2-0 win in the first leg of the round-of-16 tie at San Siro, but the holding midfielder has warned that the Catalans should not be underestimated in the return.
"Our attitude will define this match. We have to face the match in a certain way – we have to prepare with a lot of courage," Ambrosini said at a press conference.
"We have to read the game properly otherwise we will be making a big mistake. Our attitude has to be to want to hurt Barcelona. One goal would increase our chances massively.
"We have to make the most of this situation. The more you worry about things, the more you are likely to play badly.
"Barcelona are so strong that they will play as they usually do: have possession and control the match. They can't have a different attitude. They might be fiercer but we're ready for that.
"[Lionel] Messi is their most important player but not the only one. On Tuesday we have to stop the whole of Barcelona."
The match at Camp Nou is scheduled to kick off at 20:45CET.