The Swiss champions travel to Stamford Bridge on Thursday having lost 2-1 at home to the Premier League club in the first leg last week.
"Every game is difficult and we know we're playing against Chelsea, who are the Champions League winners," Yakin is quoted as saying by Sky Sports News. "They have two top players in every position.
"But we will do everything. We will be brave, we'll play with confidence and who knows? Maybe we can achieve a small wonder and we can qualify for the final."
Basel are currently three points clear at the top of their domestic league as they look to win a fourth consecutive title.