Jorge Sampaoli insists Messi will continue to be the best in the world until he retires, despite an underwhelming performance against Brazil.
Mercado's strike gave Sampaoli his win with Argentina over Brazil in Melbourne , with Brazil missing a couple of chances.
Messi could not give Albiceleste the boost they needed and Sampaoli would only praise and not criticise his captain.
"When it comes to Messi, he is and will continue to be the best player in the world until he retires," Sampaoli said at a news conference after his first game in charge of the national team.
"That is simply the way I feel about Leo.
"Overall, I appreciate the enthusiasm and the attitude of the players. We did not have a lot of time to prepare.
"We got more chances to score than Brazil in the first half and were mainly playing inside their half. We suffered a bit after the break. Brazil got some chances to level the scoring in the second half.
"It is always a motivation to beat Brazil, this game is a classic. We did not just come here to play, but wanted to win. The players fought for the jersey."