Luis Enrique's side had been hoping to bounce back from their 3-1 humbling at the hands of Real Madrid in last week's El Clasico, but Celta proved a formidable opponent.
Despite losing, Barca dominated proceedings at Camp Nou, with Lionel Messi and Neymar hitting the crossbar no fewer than four times between them.
Mascherano acknowledged that Barca had little difficulty in creating chances on Saturday, but was left concerned by their inability to implement their usual style.
"I think we played a bad game, we never got comfortable," the Argentinian told Canal Plus.
"Apart from creating scoring chances we weren't able to play our style and that's worrisome.
"Our opponents played well and we saw that some of their situations were generated from turnovers and if you want to play for this type of club you have to keep fighting.
"These were two losses [Celta and Real Madrid] in two very different games, but we didn't play well.
"[But] all is not lost, on Wednesday we have another very important game [against Ajax in the UEFA Champions League] in order to regain our confidence."
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