The European Cup holders host Arsenal at the Nou Camp tonight with the tie finely poised at 2-2 after last week's dramatic events at the Emirates Stadium.
Barcelona dominated the first hour of the first leg, racing into a two-goal lead thanks to a brace from Zlatan Ibrahimovic but the Gunners managed to haul themselves back into the tie, inspired by the second half introduction of Theo Walcott.
Brit of alright: Walcott could be used from the start in the Nou Camp
The speedy winger scored and crossed the ball which led to Arsenal's penalty and Cesc Fabregas' equaliser.
And Valdes, although supremely confident of reaching the last-four, says Barcelona must be wary of Walcott.
Back of the net: Walcott scored past Valdes at the Emirates
'Walcott showed what an important player he is for them. When he came on he showed he is attacking and his speed and talent helped them,' Valdes said.
'He is an important player for Arsenal and we will have to keep an eye on him, but they also have several other players with a lot of quality.
'We will just try to hold him up and stop him like with any player of that type who you have to keep away from the danger zones and deny space.
'He is a very fast player. For any defender it is difficult to mark him because he can go past you. There are other ways to deal with him and you have to be ready.'
Barcelona will be without the suspended defensive duo of Carles Puyol and Gerard Pique. While Ibrahimovic is out with a injury and Anders Iniesta, who missed the first leg, remains a doubt .
19-year-old Bojan Krkic is likely to lead the line for the home side ahead of the out-of-favour Thierry Henry. And Valdes is confident the Serbia forward can deliver on the big night.
Suspened: Puyol will miss the second leg after he was dismissed at the Emirates
'We will be without [key] players. But Bojan Krkic has shown howmuch quality he has. He needed to play in a game like the one againstAthletic Bilbao on Saturday, and scoring his two goals was great. Theteam already knows how good he is though.
'With every gamenow being so important we cannot afford any slip ups. We feel we shouldhave won, and deserved to win, in the first leg.
'If we analyseour chances after the first leg you would have to say it's 50-50. Butwe have home advantage and believe that gives us a bigger chance andwill be decisive. With the help of our supporters, we think it'simpossible to fail'
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