The Red Devils surrendered a 2-0 lead to tie 2-2 with Steven Gerrard's side in Australia, in a second-half performance Ten Hag branded 'unacceptable'.
United continued their impressive pre-season form with a dominant first-half performance, with Jadon Sancho's volley and a Matty Cash's own-goal - diverting Marcus Rashford's shot into his own net - giving them a healthy lead.
But there was a distinct drop in pace after the break as the rain began to teem down in a soggy Perth, and Villa took advantage.
But just like January's meeting between these sides in the Premier League, Gerrard's Villa roared back as Leon Bailey's fine solo goal was added to by Calum Chambers' stoppage-time header, ruining United's perfect pre-season record.
Ten Hag admitted the result was not a disaster, it's only pre-season after all and overall Man United have looked fit, sharp and ready for a new era in their wins over Liverpool, Melbourne Victory and Crystal Palace, where they plundered 11 goals.
But he warned his players to wise up before their league opener at home against Brighton on August 7, saying that letting a two-goal lead is inexcusable.
"In our team it was clear there was a dip," United boss Ten Hag said. "First-half really good. I think we controlled the game, good football, good change of play.
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"We had a lot of movement behind the defending line, made great goals and 2-0 up at half-time. Then a drop of focus. It can be that the guys are tired but it's never an excuse.
"They have to perform and it is what we didn't do in the second half. But the first-half really good.
"It was the focus. I don't want to make excuses about the pitch or weather. No matter what we have to perform."
Asked if the late leveller may be good for the players as it underlines that one slight drop in focus can be costly, Ten Hag said: "That is what we just said in the dressing room.
"A drop of focus is unacceptable but to the season I'm happy because now I can tell them it cannot happen.
"You have to get out of the dressing room and you have to get back to your game plan, get back in your focus and you have to get back to your performance and that is what we didn't do."
The boss added about his team's performance in the final game of the club's tour of Thailand and Australia: "Football is a sport of mistakes and we have to decrease the mistakes. That's clear.
"We will and I have seen a lot of progress in these two weeks. Also again in the first half, I saw really good football, really controlling and dominating the game.
"But we have to learn from [what happened in] the second-half. It's not possible that you give 2-0 up."