Hernandez will join his United teammates when they fly to Mexico for their friendly with Chivas.
O'Shea told manutd.com: "He's going to be a wildcard for us.
"The World Cup brought him to the attention of a lot of people, but other league teams might not know his qualities. So that should help us. He looked very sharp and I think he'll cause lots of problems for defenders and score a few goals."