Guardiola doesn't not want any assumptions made going into the second-leg in Ukraine.
He said: "The result is what it is, but Shakhtar are led by an experienced coach and I am still afraid. We have to play in Donetsk first and then we will think about Real Madrid.
"Don't believe [Mircea Lucescu] when he says the tie is over. I'm telling you: this isn't finished yet. We turned up at Real Betis [in the Copa del Rey] with a 5-0 lead and we suffered badly [losing 3-1]. I warned you all back then and I was accused of false modesty.
"We are a little short at the back as with the absences of Abidal and Puyol we're missing important players with pace and we need to replace that with other qualities.
"Having said that, the players at my disposal are physical animals and if there's one thing they know it's how to compete."
He added: "We've had three games now where Leo [Messi] has been the determining factor without scoring; the match against Madrid, against Espanyol and now this one. He can win us games in many ways."