We've selected some of the best comments below, and we want you to join in the discussion on Talking Reds.
"What a great signing! What a goal as well - wonderful goal. Can't wait to see him playing next season for United."
Posted by: xXManU4EverXx
"I'm already excited about the prospect of Old Trafford's King (Wazza) and Prince (Hernandez) playing together. We will terrorise Premier League defences next season. Hernandez was the fastest player at the World Cup, clocked at 32kmph. I hope we can beat the Scousers 7-0 with Hernandez and Rooney showing how poor their back four has become."
Posted by: kbbk
"I think he has shown that he is already good enough to be a regular starter up front with Rooney."
Posted by: Jordz
"He is definitely a good prospect. He's only 21. So let's be patient with him and not judge him or slate him if he has one bad performance. He'll need one or two seasons to adapt to life in England and the Premier League. As he starts to mature as player, he'll get even better."
Posted by: CVP1982
"Lots of people want us to sign someone, but we don't need a new striker. This kid is so good. Just look at how he scored that goal against Argentina."
Posted by: MANUTDGHANA
"I was impressed by his goal against Argentina. That one movement and finish just shows you that he is something special."
Posted by: B1GJ4V