Matthijs de Ligt was off the pitch receiving treatment when Ethan Pinnock headed home from a corner.
De Ligt was forced off three times due to a head cut, with the third resulting in Pinnock's goal.
"He was okay, that's what he told me," Ten Hag said. "That's what I asked him when he came off: 'Is there a problem?' No, he was 100 per cent there, not any time (was) there a doubt to take him off.
"I wasn't furious, but I just was asking my player how he is. He said 'it's just dry blood, it's not bleeding', so we are really surprised that he has to come off."
Ten Hag continued: "Of course, there was a huge moment, because Brentford are very good in corners, and when you then miss one of your best headers, and they take benefit from it, then of course you are disappointed."