Andoni Iraola said Nottingham Forest's penalty in the 2-2 draw at Anfield on Saturday would not have occurred if the fourth official's electronic board was working correctly.
Forest took the lead for the second time in the match on 70 minutes, when Morgan Gibbs-White converted from the spot after Neco Williams had been fouled by Alisson.
At that point, Liverpool had been trying to make a triple substitution for over two minutes - during which time the ball had gone out of play on three occasions.
Sub Ryan Gravenberch and Iraola could both be seen gesturing in frustration to the fourth official Tim Robinson, who was squatting down to two electronic boards.
According to official guidance, when substitutions are being made, the process is that the club should complete a subs card and prepare their subs board with the relevant players' numbers and, once ready, then pass this to the fourth official, who will hold it up.
"Definitely he was having some problems with the board or something," said Iraola in his post-match press conference.
Crucially, Williams' run into the box came from a quick throw taken by Liam Delap - who had come on as a sub himself, seconds before Liverpool prepared their triple change.
That was the third occasion the ball had gone out of play after Liverpool's proposed subs had gone to the touchline in readiness to come on.
Each time referee Samuel Barrott allowed play to resume, deeming Liverpool not ready to make their substitutions.
"What would have happened [if the board had been working?]," asked Iraola. "Definitely you stop because I want to sub. It doesn't happen, the quick throw-in, because we are waiting for the subs."
Liverpool's three subs - Gravenberch, Rio Ngumoha and Kostas Tzimikas - eventually came on after Gibbs-White had scored the penalty.
Liverpool went on to score an 82nd-minute equaliser through Victor Munoz but Iraola remains winless as Reds boss after two Premier League matches.
65:23 - Nottingham Forest's Liam Delap comes on for Dan Ndoye.
65:43 - Three Liverpool subs go to the touchline in preparation to come on.
66:23 - The ball goes out of play for a Forest throw-in.
One of Liverpool's three prepared subs, Gravenberch gestures towards the fourth official in frustration, with the fourth official squatting down beside two electronic boards. Iraola then speaks to the fourth official, with his arms out, seemingly asking what is going on.
66:35 - Play restarts.
It is reported by commentators that Iraola has changed who he wants to come off - deciding to leave Jeremie Frimpong on but to take off Mac Allister instead.
67:11 - The ball goes out for another Forest throw-in.
The fourth official is still squatting beside the electronic boards, trying to get them to work. Iraola gestures with his arms stretched out wide to ask why the delay to the subs is continuing.
67:21 - Play restarts.
67:38 - The ball goes out for another Forest throw-in. Delap takes it quickly to Williams to send him into the Liverpool penalty area.
67:46 - Forest are awarded a penalty when Williams is fouled by Alisson.
69:51 - Gibbs-White scores the penalty to put Forest 2-1 up.
70:41 - Liverpool's subs come on.
Liverpool boss Andoni Iraola:
"I can't explain because I don't have an explanation.
"I wanted to make those three subs three minutes before. There have been two, three, four times I think the ball has gone out and I still don't know the reason why we haven't made the subs.
"Definitely he was having some problems with the board or something.
"What would have happened? Definitely you stop because I want to sub. It doesn't happen, the quick throw-in because we are waiting for the subs.
"If I ask for the sub - and I'd already asked and I complained the previous time the ball went out and the supporters, I think they were shouting, everyone was aware of the situation.
"But I don't [know] the reason why it happened.
"The penalty, I leave you to decide if it's a penalty or not."
