Goals from Nolito and Iago Aspas saw Barcelona go down 2-0 in the opening 45 minutes at the Estadio de Balaidos, before the latter made it three after the break.
Neymar then pegged one back for Barca but John Guidetti netted Celta’s fourth goal shortly after to end any hopes of a comeback from Busquets and Co.
“After a result like that, the team feels bad. Even though we do think that we lost by a much bigger margin than we deserved,” Busquets told Barcelona’s website.
“We were better than them in the first half. We were more intense. But they scored goals at key moments.”
Wednesday’s defeat saw Barcelona surrender top spot in the league table and drop to fourth and Busquets is determined to return to winning ways against Las Palmas at the weekend.
“We have to turn the page, learn from our mistakes and prepare for the next match. We have to be more careful with counter-attacks and get things right up front,” the Spain international said.
“The ideal tonic now would be to win on Saturday against Las Palmas at the Camp Nou.
“There is a long season ahead and we had won every game up until now. We’ll get back to winning ways on Saturday.”