Following Koeman's exit, sources have told ESPN that the club will open talks with Xavi Hernandez on Friday regarding the vacancy, with the former team captain now the first choice.
However, the same sources add that Barca don't expect to have a replacement in place until after the November international break. An interim coach is set to take charge of the next three games against Alaves, Dynamo Kiev and Celta Vigo.
Koeman's position had been seriously weakened after poor results prior to the October international break, with back-to-back defeats against Real Madrid and Rayo this week proving to be the tipping point.
The 58-year-old leaves Barca ninth place in the league and in danger of missing out on the Champions League knockout phase for the first time in 20 years, with the side yet to register an away win in any competition this season.
The club said in a statement: "FC Barcelona have relieved Ronald Koeman of his duties as first-team coach. The president of the club, Joan Laporta, informed him of the decision after the defeat against Rayo Vallecano.
"Koeman will say goodbye to the squad on Thursday at the Ciutat Esportiva. FC Barcelona wishes to thank him for his service to the club and wishes him all the best in his professional career."
As for Xavi, the 41-year-old former midfield great won a slew of trophies with Barca in his 17 years as a senior player, including eight LaLiga titles and four Champions League medals. He then joined Al Sadd in 2015, becoming head coach at the Qatari club after hanging up his boots in 2019. It is his only managerial experience so far.
Along with Xavi, sources had previously told ESPN that Belgium boss Roberto Martinez and was among the candidates.
Laporta had first considered sacking Koeman in the summer and even told him he needed two weeks to try and find a new manager. In the end, due to a lack of credible alternatives, Laporta decided to stick with Koeman.
However, sources said that former Netherlands boss Koeman was upset by Laporta's actions and the relationship between the two has never been the same since.
Despite that, Laporta handed Koeman a stay of execution last month after the 3-0 Champions League defeat to Benfica in Lisbon. Since then, though, Barca have lost to Atletico Madrid, Real Madrid and now Rayo in the league.
"It shows the team's level has dipped, in terms of the balance in the squad. We have lost players with a lot of effectiveness in the final third. I think in recent years the teams at the top have strengthened and we have not been able to. That counts.
"But not today, because despite missing players we played well, but you can't say that when you don't score, not in Spain."
Asked if the result placed his job in danger, hours before he was fired, Koeman said "I don't know."
Koeman had been appointed by the previous president Josep Maria Bartomeu 13 months ago, replacing Quique Setien after Barca's 8-2 loss to Bayern Munich in the Champions League in August 2020.
He left his job with the Netherlands, paying the Dutch Football Association the compensation out of his own pocket, to take on what he had said was his "dream job."
In his first and only full season in charge, he led Barca to the Copa del Rey and a third-place finish in LaLiga. Paris Saint-Germain eliminated them in the round of 16 of the Champions League.
Koeman, who scored the winning goal in the 1992 European Cup final as a player for Barca, which earned him legendary status at the club, has seen his time on the Camp Nou bench beset by problems on and off the pitch.
Long-term injuries to Ansu Fati, Ousmane Dembele, Philippe Coutinho and Sergio Aguero, among others, have hampered his team selections, while the club have also had to dramatically reduce the wage bill during his tenure to deal with their financial problems.
Koeman also lamented the exit of star player Messi in recent weeks. He said the Argentina international had disguised many of the team's flaws and was responsible for making the players around him even better.
In terms of replacing them, Koeman has had his hands tied due to the fact the club's debt has reached around €1.4 billion. All incomings this summer, such as Memphis and Aguero, were signed on free transfers.
Instead, Koeman had turned to youth. Ronald Araujo, Oscar Mingueza and Pedri all broke through last season, with Alejandro Balde, Gavi, Nico Gonzalez and Yusuf Demir all given chances so far this term.
Since retiring in 1997, Koeman has coached in the Netherlands, Portugal, England and Spain to varying levels of success. He won two Dutch titles with Ajax Amsterdam and did a good job at Southampton, but he left Valencia and Everton after failing to live up to expectations.