Most of Hader's tweets were made in 2012, only months before his home state Baltimore Orioles were taking him in the MLB Draft.
Before the game even ended, Hader made his tweets protected.
"During last night's game we became aware of Mr. Hader's unacceptable social media comments in years past and have since been in communication with the Brewers regarding our shared concerns", the Major League Baseball said in a statement Wednesday.
MLB Deputy Commissioner Dan Halem said he had talked to Hader, and the league is expected to issue a statement on the matter in the coming days.
"Being 17 years old, you make stupid decisions and mistakes", Hader said.
Hader issued an apology in front of hoards of media after the game, saying that that comments don't "reflect on who [he] is as a person today".
"You know it was something that happened when I was 17 years old". "We always get in trouble for things you said when you were younger".
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"I was in high school". It proved a rough outing for the Old Mill High graduate, who allowed four hits and three runs in just one-third of an inning.
It wasn't a great night Hader's family or fans either. "There is no excuse for what is said".
Asked if he was anxious about facing discipline, Hader said he would live with it. It won't happen again.
"Like I said before, I was young, immature and stupid, and there's no excuses for what was said and what happened". 'If we could follow each other around with a recorder every day, I'm sure we all said some dumb stuff.
"We'll move on from it", Cain said.
When asked if he is anxious about facing any potential discipline from the league, Hader said he is "ready for any consequences". The guy I know, he's a great guy.
He surrendered a tie-breaking home run to Seattle's Jean Segura in the eighth inning.
A baseball player apologised previous year after using a homophobic slur live on TV.