Giancarlo Stanton conceded Wednesday he wasn't certain he should have agreed to the 13-year, $325 million deal he signed with the Miami Marlins.

"There was no 100 percent sure answer," Stanton said. "But the decision is made. Once you make your decision, you go all-in with it."

The deal is the most lucrative for an American athlete and the longest in baseball history.

Stanton accepted the deal after owner Jeffrey Loria pledged to put together a contending team that will finally transform Miami into a baseball town.

"You can't keep saying, 'We're going to win this year. We're going to do it this year,'" Stanton said. "I'm sick of hearing that. Everyone is sick of hearing that. It's doing something about it."