Mouctar Diakhaby and Maxi Gomez scored soft goals for the visitors as Javi Gracia's side took a point away from the Camp Nou in a 2-2 draw on Saturday.
"It was a difficult game," Koeman said at full time. "We came from behind, but we have to have more concentration. We had chances, but we didn't concentrate at certain moments.
"They defended well. We made the effort in the second half to win [the game], but we didn't create as much as we'd wanted.
"We had an inconsistent game. We lost possession in areas that were dangerous. I played with three [centre-backs] to control the game better."
When probed as to why his players might have suffered from inconsistency and a lack of concentration, the Barcelona boss had one suggestion.
"The age of the players also has an influence," Koeman added. "It can generate doubts. The youngsters have to mature."
One of those youngsters who did make the difference was Ronald Araujo, who scored his first goal for the first team.
"It's great for a defender to score," Koeman said. "[Araujo] worked well. It's a shame his goal wasn't enough for the three points."
Koeman added: "We still haven't said goodbye to the title.
"There are a lot of points left [to be won]. It's a very tough season. In two months, everything could be different. We have to fight to make up the points difference."