Coach Giovanni Trapattoni rued some poor defending and what he regarded as a refereeing error after his Republic of Ireland side lost for the first time in 15 games, going down 3-1 to Croatia in their Euro 2012 Group C opener in Poznan, Poland, on Sunday.
A Mario Mandzukic brace and one from Nikica Jelavic handed the Croatians a winning start with Sean St Ledger scoring for Ireland.
Trapattoni felt the Irish had been hard done by as he believed that the Jelavic goal was "clearly offside" and changed the course of the game coming as it did just moments before the break - however TV images clearly showed the ball came off Stephen Ward which played Jelavic onside.
"The first goal was our mistake," said Trapattoni.
But "the second goal was clearly offside," the Italian insisted - though he admitted that a major post mortem was needed after the first Irish loss in 15 outings.
"Tomorrow (Monday) we'll take the time to clarify what happened in the first half, because yesterday (Saturday) I said we were ready."
The Irish were facing an uphill battle after conceding the first goal inside the opening three minutes, Mandzukic doing well as he regained his footing to head in a cross from captain Darijo Srna, his effort appearing to catch veteran Irish goalkeeper Shay Given in no man's land as he scrambled back in vain.
"Maybe Shay didn't see the ball, because he had three or four players in front of him," Trapattoni ventured.
Veteran Given - Ireland's most capped player with 123 - had been a doubt because of a knee injury but after being able to train fully on Friday suffered no reaction and was cleared to play in Sunday's match giving Trapattoni a boost.
But although St Ledger headed the equaliser the Croatians restored their lead just before the break when Ward miscued his clearance and Jelavic was able to lift the ball over Given from close range.
Three minutes after the break, Croatia made it 3-1 when the ball went in off a diving Given's head after Mandzukic's header hit the post.
Ireland then had a penalty claim denied by Dutch referee Bjorn Kuipers after an apparent foul on skipper Robbie Keane.
With Spain and Italy to come Irish chances of reaching the last eight look slim.
"Spain is another game. We have to think very much. I must take all the players in confidence - if I decide to make changes now, it will not be because of playing bad or not deserving to play," said Trapattoni.
"We have to believe. Every game in this Euro could be a surprise. We have to believe this," he concluded.
Keane said meanwhile: "Two goals killed us. The one before half time and the one after that, that was the killer blow and it was always going to be uphill from there."