Messi said Madrid deserve credit for winning all 10 games since La Liga's resumption in June to prevent Barca from making it three titles in a row, but he criticised his side's performances and called for reflection across the whole club.
"We didn't expect or want it to end in this way, but it's a reflection of our season," Messi told Movistar after Osasuna ended Barca's 42-game unbeaten run at Camp Nou in the league.
"[We've been] an inconsistent, weak team that has been beaten in intensity and desire. It has been easy to score goals against us.
"Madrid did what they had to do. They haven't lost a game since the restart, which deserves credit, but we have helped them by dropping points we should not have dropped.
"We must be self-critical, starting with the players, but across the whole club. Madrid won all their games but we are Barcelona and we're obliged to win every game. We have to look at ourselves, not the opposition, and the performances in recent games have left a lot to be desired."
Barca could still avoid a first trophy-less season since 2007-08 with success in the Champions League when Europe's premier competition resumes next month.
However, Messi says huge changes will be needed before they host Napoli in the second leg of their last-16 tie on Aug. 8 following a 1-1 draw in Italy in February.
"I have said before that if we continued playing as we were it would be difficult to win the Champions League and now it's clear it wasn't even enough to win La Liga," the Barca captain added.
"We have to change a lot because if we don't, we will lose against Napoli as well. We need a little time to clear our heads and then to think about the Champions League, which starts from zero."
Messi is out of contract in 2021 and a report in Spain earlier this month suggested he is considering his future at the club beyond then. Sources subsequently confirmed to ESPN a number of issues on and off the pitch have upset him.
Missing out on the title to Madrid is unlikely to help convince him to stay and, while he didn't want to comment on his own future, he acknowledged the club's supporters have a right to be unhappy.
"The fans are angry with all that's gone on this season and it's normal, we are also angry," he said. "It's normal that they feel like this and that, after the defeats against Roma and Liverpool [in the Champions League in 2018 and 2019], they are losing patience because we're not giving them anything."