The Arsenal midfielder connected with the cheek of Niko Kranjcar in an off-the-ball incident on the hour and was dismissed by referee Tommy Skjerven after consulting his assistant. Gunners boss Arsene Wenger was in the stands to witness Pennant's punch, which came as he is about to seal a two-month loan move to Leeds.
Back in April, Pennant was sent home from the U-21s' camp after breaking a curfew before the clash with Turkey. Platt told Pennant after that incident he had not acted "in a professional manner" and said tonight: "On-the-field is different from off-the-field, but Jermaine's got to learn about everything that's happening in his career. "It was retaliation for something but the boy deserved to be sent off tonight and he's held his hands up.
"He hasn't been sent off that many times in his career, certainly not for retaliation.
"But you've got to learn from things like that, although that's not an excuse. He and we must be more professional."
Pennant is expected to receive a three-match suspension from U-21 and senior international friendlies. This was a night to forget for Platt but Pennant is still be available for next month's vital European Championships qualifiers against Macedonia and Portugal.
Platt was keen to emphasise that Pennant's dismissal should not mask the deficiencies in his side's performance. Hammers striker Jermain Defoe had an equaliser wrongly ruled out for offside after 37 minutes.
That would have levelled Goran Ljubojevic's excellent 11th-minute opener.Ljubojevic netted again seven minutes before Pennant's aberration, reacting first to a loose ball inside the six-yard box and hammering home.
And Croatia left-back Danijel Pranjic completed the victory with a quite brilliant chip in injury-time. The visitors also hit the crossbar twice when England goalkeeper Matt Murray appeared out-of-position, although Pennant hit the post before his dismissal.
Platt said: "Jermaine's sending-off didn't help our situation, but I wouldn't be stupid enough to say it off was the reason we lost the game. We have to all take responsibility.
"It's an extreme disappointment and it wasn't a club performance, we didn't do things players do week-in, week-out at their clubs.
"Pulling on an international shirt on means you must do that but people might have been trying to impress.
"I told my players it wasn't acceptable to us as a country to get beaten 3-0 at home in an international.
"We have to become brighter at this level and understand what not losing games is about.
"And if it's the kick up the backside we need to win the two games with Macedonia and Portugal, I'll be delighted."