Defeat to Osasuna on Thursday ended any lingering hopes Barca had of catching Madrid and prompted an angry response from Messi, who called the team's performances throughout the season "weak" and "inconsistent."
Barca reacted by beating Alaves 5-0 in their final league game of the season on Sunday, with Messi scoring twice as a disappointing campaign ended on a high note.
"We were self-critical and took a hard look at ourselves from the inside [after Osasuna], which is what we needed to do," Messi told Movistar after the Alaves win.
"We realised a number of things, firstly that we have not had a good season in terms of performances or results -- as can be seen. I think today was a step forward in terms of attitude and commitment, which is what we must always have.
"Things can go one way or another but I don't think we should be beaten for desire. We have to go out with the same desire as our opponent, or even more, and the performances will stem from that."
Ansu Fati, Luis Suarez and Nelson Semedo were also on target at Mendizorroza in a display that will reignite belief Barca could win the Champions League next month, with young midfielder Riqui Puig impressing with two assists.
They host Napoli at Camp Nou in the second leg of their round-of-16 tie on Aug. 8 having drawn 1-1 in Italy in February.
Messi says everything that needed to be said has now been said and called on his teammates to do their talking on the pitch now as Barca look to avoid a first trophy-less season since 2007-08.
"We're all conscious of the fact we have to be much more consistent," Messi added. "As the coach [Quique Setien] said the other day, there were games when we were very good in spells, but we have to maintain that over 90 minutes.
"I think we need a little calm now, a break to clear our heads and come back with more desire than ever. We have said everything that needed to be said. We've been self-critical.
"Now it's time to work hard and give our all. There's nothing left to talk about, we just have to show that things can change and today was an important step forward in that."
Messi's brace against Alaves took him on to 25 league goals for the season and edged him closer to a fourth successive Pichihchi Trophy -- the award given to La Liga's top scorer.
Only Real Madrid's Karim Benzema, who has scored 21 goals and has one game left, stands in his way of what would be a record-breaking seventh Pichichi. Legendary Athletic Bilbao striker Telmo Zarra also won six.
"I've always said that individual prizes are secondary," Messi said. "It would be significant to win it seven times, but I would have liked it to be accompanied by the league title."