The Red Devils boss was left unimpressed by Forson, 19, taking exception to a challenge from Julian Ryerson.
The winger kicked out at the Dortmund defender before squaring up to Karim Adeyemi in an incident which led to Ten Hag hooking Forson after only 38 minutes, before giving him a lengthy dressing down on the touchline.
“When there is a VAR, he gets sent off,” Ten Hag explained post-match.
“He played a good game and the opponent gets annoyed with him so that is a learning [curve] but don’t take it into your own hands.
“You can’t be the judge, you have to play football, you have to beat your opponent, you have to net and that is the best way to react.”
Forson was not the only player to underwhelm Ten Hag in Las Vegas as United fell to a 3-2 defeat.
Donyell Malen scored twice in two minutes to turn the friendly-around after Diogo Dalot’s goal, and Youssoufa Moukoko took advantage of defensive errors to score the winner despite United hitting back through Antony.
“There are no excuses,” said Ten Hag.
“We have to perform no matter what and then you have to be ruthless and win games because we are Man United and the fans are expecting from us wins.”