Barca president Laporta eventually dismissed Koeman after the 1-0 loss in Vallecas in midweek with Barca ninth in the league after just two wins in their last seven games.
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Koeman, who won the Copa del Rey in his first season in charge at Camp Nou, had also overseen a disastrous start to Barca's Champions League campaign, with the Catalans taking just three points from their first three European outings.
Sources have told ESPN that Al Sadd coach Xavi Hernandez has been lined up to replace Koeman, although Laporta wouldn't comment on negotiations to bring the club legend back to Camp Nou.
"The decision quite possibly arrived late," Laporta said in a news conference on Friday after sacking Koeman on the flight back from Madrid on Wednesday.
"We should have removed him earlier but it's easier to analyse things with the benefit of hindsight. In September [before the international break], when we backed him, we felt he deserved to be shown some confidence and be given the chance to get the injured players back.
"I take responsibility for not making the decision before. We beat Valencia and Dynamo Kiev but then came a series of bad results. We were drifting dangerously and we risked losing touch in all competitions.
"We're grateful to Ronald because he took on the job in a very difficult moment. He's a club man and this will always be his home. For us, he's a Barca legend but if he has not continued in the role, it's because of results. The situation was unsustainable and we thought not acting could weaken our chances of achieving our objectives, which are to fight for every trophy."
Xavi, who left Barca in 2015 for Qatar, where he's been coaching since 2019, is the favourite to replace Koeman, with sources telling ESPN that talks for him to return to the club are at an advanced stage.
"I'm going maintain my silence on the options we have on the table because it could damage the appointment process," Laporta said when asked about Xavi.
However, the Barca president did reveal that he's been in regular contact with Xavi in recent months and that he believes the former midfielder, who made 767 appearances for the club as a player, is ready for the job.
"Xavi is evolving well as a coach," Laporta said. "I have great references from people that know more than me in that regard. I speak with him often. We have and have always had a great friendship, despite what's been said by some media outlets.
"I know his opinion, what he thinks about the squad we have and what we need to do. My opinion of him as a coach is very good. I haven't seen [many of his games] with Al Sadd, but I have watched Barca games with him speaking on the phone.
"But I prefer not to speak about Xavi. He's an option but we have several."
Sources explain that the immediate appointment of Xavi has been held up at the Qatar end, with Al Sadd keen for him to oversee their next two games. They face Al Ahli on Saturday and then Al-Duhail in a match that pits the top two against each other in the Qatar Stars League before a one-month break.
In the meantime, B team coach Sergi Barjuan has been named as Barca's interim boss, with Laporta confirming he will be in charge for Saturday's game against Alaves and Tuesday's Champions League trip to Kiev.
Barjuan, speaking alongside Laporta, confirmed that Ansu Fati will once again sit out the Alaves game with a knee injury, but revealed Ousmane Dembele could feature for the first time this season after recovering from a hamstring problem.
Barca are ninth in La Liga with 15 points from 10 games, nine points behind surprise early leaders Real Sociedad, who have played a game more.