The Blues travel to the Nou Camp on Tuesday protecting a one-goal lead from the first leg at Stamford Bridge.
Alonso's Madrid beat Pep Guardiola's men in Saturday night's El Clasico to go seven points clear at the top of La Liga.
The Spaniard believes that Barca's invincibility in recent years is starting to wane.
"We know that over the last few years, Barcelona have been so successful, that they have been playing great, wonderful football. But they are not unbeatable," The Mirror quotes Alonso as saying.
Alonso moved to Madrid from Liverpool in 2009.