Athletic Bilbao’s Markel Susaeta (second left) is challenged by Barcelona’s Sergio Busquets (left) and Neymar.

Barcelona’s 2-1 loss at Ajax Amsterdam in the Champions League last week was down to a lack of intensity while Sunday’s 1-0 reverse at Athletic Bilbao in La Liga exposed their relative physical frailty.

Those were two of the explanations coach Gerardo Martino and the players offered after the Spanish champions suffered their first two defeats of the season during a miserable week that saw them surrender their three-point lead over Atletico Madrid at the top of the La Liga.

Argentine Martino, in his first campaign in charge since taking over from Tito Vilanova, will need to quickly identify and fix whatever it is that is ailing his side or risk missing out on major silverware for the first time since the 2007-08 season.

Barca are through to the last 16 of Europe’s elite club competition and can secure top spot in Group H if they avoid defeat at home to Celtic on December 11.

They also remain top of the domestic league on goal difference but there is a sense that when the season reaches its climax around April the flaws that were evident over the past week will be even more cruelly exposed.

On current form, a disaster similar to the 7-0 aggregate drubbing they received at the hands of Bayern Munich in the Champions League semi-finals last term would not be a major surprise.

Barca do have a number of key players sidelined by injury, including goalkeeper Victor Valdes and World Player of the Year Lionel Messi, who is not due back until the middle of next month.

But with players of the calibre of Neymar, who fluffed a number of chances on Sunday, Cesc Fabregas, Andres Iniesta and Alexis Sanchez, Barca should be winning games against sides like fifth-placed Bilbao comfortably, at the very least avoiding defeat.

Midfielder Sergio Busquets conceded that Bilbao had been more effective at closing the Barca players down in their own half at the San Mames.

“They are generally stronger than us, we are not a team like Athletic for example with all the players pressuring up the pitch,” the Spain international added.

“We have a number of players out injured and there is no need for alarm.”

Barca midfielder Andres Iniesta felt his side were relatively comfortable until the goal, saying on his club’s website: “We had the game under control, but the goal really hurt us.

“San Mames is always a tough place to come to.

“We put in a good first half and we played well at times after the break, but the goal really shook us and although we really tried, we couldn’t come back.”

Javier Mascherano added: “Nobody likes to lose, the defeat hurts, but let’s not forget we are still leaders.

“We’re going away sad. We were more intense than against Ajax, but that wasn’t enough to give us the win.

“We have to put right the things we have done badly and we must be self-critical, but we’ve done things well too.

“We have to win the games that are left this year, beginning with the matches against Cartagena and Villarreal.”

Martino also called for calm and said there had been nothing in Barca’s performance that was a particular cause for concern.

“There is cause for concern when the team does not know what to do and I did not see any evidence of that,” he told a news conference.

Barca’s next outing is a King’s Cup last 32, first leg at third-tier Cartagena on Friday before they host Celtic at the Nou Camp the following Wednesday.