West Ham midfielder Mark Noble was not convinced by the penalty that gave NK Domzale an early lead at Stozice Stadium.
However Noble admitted the Slovenian minnows deserved the victory that has left West Ham's Europa League future in doubt.
The Hammers enter the second leg of their Europa League third-round qualifier with a 2-1 deficit after Matic Crnic pounced in each half, the opener delivered from the spot after debutant Havard Nordtveit was adjudged to have handled the ball.
Captain Noble equalised with a penalty, but West Ham were comfortably second best and their skipper has demanded an improvement in their first outing at the Olympic Stadium next Thursday.
"I don't think it was a penalty in the first half, but you have to get on with it and we ended up losing the game," Noble told BT Sport.
"Playing at the Olympic Stadium is something to look forward to, but obviously this result isn't ideal.
"It's our first game there and we've got to be positive - I'm sure the fans will be behind us - and win the game.
West Ham 2016/17 pre-season fixtures
"You're going to miss a player like Dimitri Payet of course, but that's no excuse. We had a good enough team to come here and win but we didn't do that.
"These games are tough, but we didn't play well. They played well and deserved it."
