Kyle Lafferty felt Northern Ireland "didn't turn up" after suffering a 1-0 defeat against Poland in their Euro 2016 debut on Sunday.
A second-half goal from Arkadiusz Milik proved enough to settle the Group C opener in Nice, meaning Northern Ireland suffered their first defeat in 13 international matches.
Michael O'Neill's side were defensively strong but struggled to pose too many problems to a Poland team that had never previously won a game at a European Championship finals.
"I think we were overawed," Lafferty told BBC Sport.
"That was not the team that has got us here, the team that Michael O'Neill organised that had the fight and determination to get us through to these finals. We lacked a lot of that and have come away with no points.
"We knew it would be tough, but I think the whole performance was not a Northern Ireland performance and on Thursday we will go and put it right."
Lafferty promised Northern Ireland will produce a better performance when they take on Ukraine on Thursday.
"We can change the fight and determination," the forward added.
"We didn't turn up, Michael [O'Neill] will have a word with us, he will speak the truth. We will take note of it and will have a more determined and heartful performance on Thursday."
Captain Steven Davis admitted they simply hadn't been good enough to get anything out of the game at the Stade de Nice.
"It's a disappointing evening. We have to braver on the ball but it's a real learning curve for us," he said.
"We always knew it would be difficult but we weren't good enough."