MLB announced the discipline Friday, two days after the game.
The commissioner's office said in a statement that it interviewed Prieto and examined his watch and cellphone. It said it "found no evidence that Mr. Prieto used the Apple Watch or cellphone for any purpose in the dugout, nor any baseball-related communication on either device, during Wednesday's game."
Prieto and the team were fined because he violated a regulation, and MLB will donate the fine money to hurricane relief efforts in Puerto Rico.
Last month, the Boston Red Sox were fined by MLB for using an Apple Watch to relay signs to hitters.