Ernesto Valverde says he understands why some Barcelona supporters whistled his team following the disappointing goalless draw with Slavia Prague in the Champions League. Barca would have taken a major step towards the knockout stages of the competition with victory at the Nou Camp. But Valverde's side, who struck the woodwork through Lionel Messi, were unable to break down the Czech champions in a game which saw both sides have goals disallowed for offside. The stalemate followed Saturday's surprise 3-1 loss at Levante to increase the pressure on head coach Valverde. "It's okay for people to demand more," said Valverde after the game. "You always like to win. Today, the truth is that we have not been right with the clear chances that we have had. "They, without creating clear opportunities, caused danger when they stole the ball from us because they came out very fast against us. We couldn't convert the dominance we had into opportunities." Barcelona remain top of Group F, although Borussia Dortmund reduced their advantage to a single point by coming from two goals down to beat Inter Milan 3-2 in Germany. After two poor results, Valverde admits his team must quickly respond, starting with Saturday's LaLiga game against Celta Vigo. "The other day we lost against Levante and we understand that it wasn't one of our best games," he added. "Neither that one nor today's. We haven't played two perfect games and we know that there is a lot of pressure towards the team and we have to bounce back." A frustrating evening for the Spanish champions was compounded by an injury to left-back Jordi Alba, who had to be substituted just after half-time. "Jordi has a hamstring problem and we'll see what the doctors say," said Valverde. The match was the first time Barcelona failed to score at home in any competition since February 2018. Slavia goalkeeper Ondrej Kolar twice saved from Messi and also repelled a powerful header from Gerard Pique. Defender Pique called for patience from fans and is not overly concerned by the current situation at the club. "I've been at Barca for many years and when people want things to go better than they are then there's criticism," said the centre-back. "We'll try and sort things out, to improve, but it's time for a cold-blooded, calm-headed analysis. "We are leaders in the league and in this group. We can improve the play and the results but things aren't that bad. I'd just ask the fans to show us some patience. We are giving everything to find the way to improve our play." Slavia keeper Kolar said: "It is hard to believe that we managed to get a point here, a very precious point. If somebody told me that I would not concede a goal, I would think they had gone mad! Barcelona had scored against every team here this season."