The incident caused some debate, with similar challenges going unpunished over the weekend, and Sky sources understand Liverpool are appealing the length of Mane's three-match ban.
Although there is little time for recovery ahead of Thursday morning's Champions League opener against Sevilla, Mignolet says a positive result can quickly put the Merseysiders back in the right mentality.
"I have seen a couple of matches and they are one of the best teams in Europe. He was shocked about the situation", added the Reds boss.
Now he needs to forget about what has gone and perform at his best for Liverpool again.
"I am not concerned in the long term", Klopp said.
"The only thing we didn't do a year ago was win the big games", De Bruyne, who set up goals for Sergio Aguero and Gabriel Jesus, said after Saturday's match. It was really bad, but we have to take the blame for everything that you saw on Saturday. I don't blame the ref for the decision. That means he could be free to play in the Champions League if his facial wounds heal.
Jurgen Klopp had initially said they were unlikely to appeal the ban.
After avoiding Real Madrid, Barcelona, Bayern Munich and Paris Saint-Germain, Liverpool have to be delighted to end up with Sevilla, Maribor and Spartak Moscow.
Klopp stands by Liverpool's failed Mane appeal
"You go from being on the bench watching the game and within seconds you're out there playing".
Mane will next be able to return to Premier League action October 1 against Newcastle.
There is no let-up either: Koeman's men, who travel to Italy in the Europa League on Thursday, go to Manchester United on Sunday.
There were serious questions regarding the Liverpool backline, the center-backs in particular, heading into this season.
But former Liverpool defender Steve Nicol told ESPN FC the full punishment was warranted.
Eyes in particular will be on City playmaker Kevin de Bruyne, whose sublime form played a major part in the Liverpool drubbing, and who is sure to sniff out and exploit weaknesses in Feyenoord's defence.
It was a miserable Liverpool debut for Alex-Oxlade Chamberlain, whose last game for Arsenal was in a 4-0 rout by Klopp's team before the worldwide break.