Luiz put the Blues ahead in the Premier League clash at Anfield with a quick set piece that took Mignolet by surprise when he wasn't looking.
Speaking on BT Sport (h/t Tom Farmery of Mail Online), Gerrard didn't pull his punches over Mignolet, who didn't realise the whistle had been blown by referee Mark Clattenburg.
"Surely after the game he is going to hold his hands up for this," the former Liverpool captain said. "He's not read it, and he's trying to push his back line out when he should have his eye on the free-kick taker, because the whistle clearly gets blown, everyone can hear it, and Simon's just in no man's land basically."
Mignolet redeemed himself somewhat with a save on a Diego Costa penalty kick in the second half, but Liverpool legend Gerrard, who began his new role as a youth coach at the club on Wednesday, had more to say.
"He's concentrating on something he shouldn't really be concentrating and is trying to lift the line when he should be leaving that for the lads to do themselves and focus on the free-kick taker," Gerrard said. "Simon's at fault for this goal."
Twitter users were quick to point out Mignolet's error:
Mignolet's mishap will likely lead to fans questioning why he is in the team, and the goalkeeping position is arguably the greatest area of concern for Liverpool manager Jurgen Klopp.
Klopp began the season with Mignolet as his first choice before dropping him for new signing Loris Karius in October. However, Karius suffered some high-profile errors himself, notably fumbling a free-kick against West Ham United and gifting Bournemouth a 4-3 injury-time winner in December.
Those mistakes led to the German boss choosing to reinstate Mignolet.
Such uncertainty in the No. 1 position has arguably been a key factor in Liverpool's shakiness in defensive situations this season.
Indeed, it has been a boost to opposing teams, and Klopp was particularly irked by Bournemouth's admission that they targeted Karius as the Reds' weak link when the two sides met in December, per Andy Hunter of the Guardian.
Karius' displays and a subsequent interview where he defended himself also drew a response from a former Anfield legend, Jamie Carragher, who said, "Just shut up and do your job," per Sky TV (h/t David Prentice of the Liverpool Echo).
For all their attacking flair, Liverpool have been found wanting on several occasions at the back, perhaps most notably in their recent 3-2 Premier League defeat to Swansea City.
A leaky back line cost the Reds the title in the 2013-14 season under Brendan Rodgers. And while there have been signs of progress, such as the emerging centre-back understanding between Joel Matip and Dejan Lovren, it's likely to be an issue that costs Klopp's men the chance to catch Chelsea.
For now, Klopp needs to show full faith in one of his goalkeepers.
Should he decide to make another change and bring Karius back off the substitutes' bench to replace Mignolet, it will arguably be a key move and will determine truly whether Liverpool can cut the gap and kick-start their title challenge.