The legendary Argentine scored for Barca but they fell to a 2-1 defeat to Osasuna - which was compounded by the Catalans surrendering the La Liga title to arch-rivals Real Madrid - and Messi felt the game summed up their season.
He told Movistar: "To be honest, this is not the way we wanted things to end, but tonight's game is a microcosm of our entire season.
"We've been too weak and vulnerable. It's not normal that another team beats us for intensity and desire. We dropped too many points along the way and that sums up our season.
"Real Madrid did what they had to do after the restart and didn't drop any points, we helped them along the way by losing points that we should not have let slip.
"We need to take a look at ourselves. We are Barcelona and we should be winning everything."
Messi also warned the team they must improve if they are to stand any chance to win the Champions League, which will be resolved in August after being suspended due to the coronavirus outbreak.
The 33-year-old star said: "I'm telling you that if we continue in this manner, we will not advance in the Champions League as we weren't good enough to retain the La Liga title.
"We're going to need to change plenty if we want to deliver and we also need to take a long hard look at ourselves.
"We lost the title due to our own shortcomings and not because of what Real Madrid did as many people are claiming."