Stephen Kenny said his Republic of Ireland side were denied a "cast-iron" penalty in the opening 3-2 World Cup qualifier loss in Serbia as the absence of VAR only increased his frustration.
With the scoreline at 1-1, Kenny was convinced striker Aaron Connolly was clipped by Serbia's Stefan Mitrovic.
"The player has gone through him to win the ball," said a disappointed Kenny.
"How could there not be VAR if there is going to be on Sunday? There is for some matches and not others."
Italian referee Davide Massa waved away the Republic's penalty appeals as he instead awarded a corner, judging that Mitrovic had won the ball fairly.
Defeat in their opening qualifier against a team expected to be the Republic's main opposition for second spot in Group A behind European champions Portugal means the Republic will badly need a home win against Luxembourg on Sunday.
Kenny is still awaiting his first win as Republic boss after nine games even though his side took the lead in Belgrade.
Browne goal ends 11-hour goal drought
Alan Browne's 18th-minute header ended a Republic goal drought of more than 11 hours but Dusan Vlahovic levelled five minutes before the break and two goals by substitute and Fulham striker Aleksandar Mitrovic put the hosts in control before James Collins' late reply.
"They've scored with their first chance," added the Republic manager.
"They've been ruthless with their finishing on the night so we're really disappointed to lose that game."
Serbia's accuracy in front of goal was best demonstrated by Mitrovic's superb chip over young Republic keeper Mark Travers shortly after the controversial penalty incident.
"Overall I can't believe we conceded three goals because it didn't look like that type of match that we would concede three goals," continued Kenny.
"They had a period in the first half where they pushed us back. Tactically (Dusan) Tadic dropped into midfield and overloaded us a little but but they didn't penetrate.
"We were comfortable defensively. Seamus Coleman marshalled his defence brilliantly.
"3-1 down was a kick in the teeth. You were wondering how you were 3-1 down."
Kenny defends keeper Travers
Mitrovic caught Travers off his line with his sensational finish but Kenny defended the Bournemouth keeper.
"Mark has been brilliant for us overall. He made a good save at the end of the first half for us as well.
"The attitude of the team was brilliant.
"We worked well and got a goal back. We rallied all the substitutes probably made an impact and we kept going to the end but we've come out on the end of a narrow defeat."
Coleman's return to Republic duty for the first time since October 2019 saw Kenny opt for a back-three with Matt Doherty operating as a right wing-back.
The Republic looked to have a good shape in the opening quarter before Tadic began to pull the midfield strings.
"We have to be flexible. We can't tie ourselves to one system," added Kenny.