The Barcelona star could not explain why his side dropped the pace in their 1-0 win over Jamaica and says they cannot afford a repeat in the knockouts
Gonzalo Higuain hit the only goal of the game to ensure Argentina finished top of Copa America Group B.
The 14-time champions dominated the Caribbean side from the outset but were unable to build on their 11th-minute lead and their captain has warned they must improve in the knockouts.
"In the second half our pace dropped, we played much slower," said Messi, who was making his 100th appearance for his country.
"We did the same against Paraguay. We struggled to find spaces, the pitch was slower... I do not know why, but it is true that we ran slower.
"In the knockout matches we can't let that happen again."
Sluggish second-half showings are becoming a concern for Argentina, who threw away a two-goal half-time lead against Paraguay in their Copa America opener last week to draw 2-2.
Argentina, without a trophy at senior level in 22 years, will now meet either Ecuador or the third-placed team from Group C, which could see them go up against Colombia or Brazil in the quarter-finals.
"Chile are playing very well, and Colombia showed against Brazil that they are also a candidate," added Messi.
"Every opponent will be difficult."