Qatar-owned Malaga snatched a 2-2 draw in last week's first leg at the Nou Camp and Barca went on to suffer their first La Liga defeat of the campaign when they let slip a two-goal lead and were beaten 3-2 at Real Sociedad on Saturday.
They have also been unsettled by last week's announcement by goalkeeper Victor Valdes that he was not going to renew his contract when it expired at the end of next season.
Malaga, like Barca through to the last 16 of the Champions League, have suffered a dip in form in recent weeks but have a golden chance to secure a probable Cup semi-final meeting with Real Madrid, whom they beat in La Liga at the end of last year.
Real have one foot in the last four after they beat Valencia 2-0 in the first leg in Madrid and play the return at the Mestalla on Wednesday having won 5-0 there in La Liga on Sunday.
Knocking out Barca would send shockwaves around the world, according to Malaga defender Martin Demichelis.
"This past weekend we saw that Barcelona are not unbeatable and that it's even possible to come back and win from 2-0 down against them," Demichelis said.
"We have proved that we can cope with difficult situations or tough games," the Argentine added.
"They may be angry and fired up and that means it could be harder to play them.
"Hopefully some seeds of doubt have been sown but they are a team used to going out and winning every two or three days."
On the other side of the draw, 2010 runners-up Atletico Madrid are in a strong position ahead of their second leg at Real Betis, having beaten the Seville-based club 2-0 last week at the Calderon.
If they can protect their lead, Diego Simeone's side will meet Sevilla or Real Zaragoza in the semi-finals, who drew 0-0 in Zaragoza in the first leg.
"Betis are a difficult and tough opponent," Simeone said on Atletico's website.
"They are showing their potential in La Liga and we will go there with humility and hope to respond well to the challenge," the Argentine added.
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