Messi has scored four goals for the South Americans so far, while goalkeeper Romero made a number of crucial saves during his country's 1-0 victory over Iran.
Atletico Madrid manager Simeone has been delighted with the form of each player, and believes that their displays will improve the confidence of their teammates ahead of the knockout stages.
"Lionel Messi has stepped up in the important moments to win games for Argentina at this World Cup," Simeone told Goal.com. "He is a player who offers something different to Argentina, but the team needs to grow as a unit and I'm convinced they will do that as the World Cup moves into the knockout phase.
"I was also very happy with the performance of Sergio Romero against Iran. In a World Cup, when you have a player like Messi who can win you any game, plus a goalkeeper who is growing with each match, that's very important.
"At a tournament like this one, if your goalkeeper is on form and your most decisive player is also on form, that gives you confidence at both ends of the pitch. It's a weapon all teams would like to have."
Argentina will face Switzerland in the last 16 in Sao Paulo next Tuesday.