Messi has been central to Barcelona's astounding success in the past few seasons, but he has failed to show the same kind of scintillating form for his country.
"We must let him be happy. We must let him play completely freely on the pitch," Sabella, 56, said at a press conference after arriving in Bangladesh on the second leg of the side's short South Asian tour.
The new coach said he has struck up a good relationship with Messi, who was named captain ahead of the two friendlies, one in India against Venezuela last week and the second in Bangladesh tomorrow against Nigeria.
Argentina won the match in Kolkata 1-0, with a Messi corner setting up the goal for Nicolas Otamendi. Sabella said Messi would repeat his prolific scoring feats at Barcelona for Argentina once he had settled in as national captain.
"I am waiting for him to score. We must give him the tranquility. He is the best player in the world," Sabella said, as his side prepares for 2014 FIFA World Cup™ qualifiers starting next month.
Argentina disappointed home fans in the Copa America in July when they lost to eventual winners Uruguay in the quarter-finals. The failure cost Sergio Batista his job as national coach and led to Sabella's appointment.
Revenge not needed
Sabella would not reveal his team for the game in the Bangladeshi capital Dhaka but said the side had no grudge against the opposition, who beat Argentina's second team 4-1 in Nigeria in June.
"I don't think revenge is a good word. It's not good for human beings. It's not good for anyone. It's a football game and we'll try to win," he said.
"It's a different game. It's a different country. A neutral country. We don't have too much time to prepare. I am a new coach. I just started to know all the players and the players started to know me," he added.
Messi's arrival in Bangladesh - which has never hosted top-class international football before - drew scores of fans wearing Argentinian jerseys to the airport, while many others camped outside the team hotel.