Sunderland were on course for a 0-0 draw at Old Trafford before Wayne Rooney scored from the penalty spot in the 66th minute.
East awarded the penalty after Radamel Falcao went down in the box.
Replays showed John O’Shea had pulled the Colombian back and Brown swept around the side of the striker as he fell to the floor.
But, to the astonishment of Poyet and the Sunderland players, referee East showed a red card to Brown, rather than O’Shea.
In a statement released via the Professional Game Match Officials Limited (PGMOL), East insisted it was not a case of mistaken identity and he did in fact mean to send Brown off as he believed the former United right-back committed a foul on Falcao.
But Poyet was puzzled by the decision.
“He (East) told the players there were two fouls. One committed by John O’Shea and one committed by Wes Brown,” the Sunderland boss said.
“He gave the Wes Brown one so that’s why he sent him off. Wes Brown didn’t touch anyone, so I don’t know what he saw…
How can you give a penalty that didn’t happen?”
Poyet said he will wait until the submission of the report before deciding whether to appeal Brown’s three-match ban.
“We need to realise there is a report and there’s something that referee saw which we think didn’t happen,” the Uruguayan said, calmly.
“So with everything on the table we will make decision whether to appeal.”
Poyet’s temperature rose when he was asked about the standards of refereeing in the country.
The former Chelsea midfielder clearly thinks referees are not owning up to mistakes and believes the whole process of improving officiating standards is not good enough.
“That showed it doesn’t matter what you say, they’re going to say what they want to support the referees and that is not honesty,” Poyet said.
“They need to be more honest – the ones who are checking on the referees.
“We are human beings. I made a decision today. I didn’t change early enough to stop that penalty happening. That was my decision and I can make a mistake.
“So they need to accept that as well and not hide. Because at the moment they’re hiding a little bit.
“The referees have to get better. There are too many (mistakes) every day.”