Twenty-five miles, 18 places and one division may separate Nottingham Forest from Leicester City but ultimately it was the crazy two minutes in the first half that exemplified the difference between these East Midlands rivals. That was the time between goals from Philip Zinckernagel and Brennan Johnson as Forest, from the Championship, deservedly knocked out FA Cup winners for the second round in succession to land a fifth-round tie at home to Huddersfield Town. The FA has confirmed it will investigate the unsavoury scenes that followed Joe Worrall’s goal nine minutes later as a fan from the Leicester end found his way on to the pitch and took two punches at the celebrating Forest players before stewards took him to ground. Leicester have confirmed they will ban him for life. It is eight years since these teams last met but, with some of the form Forest are producing under Steve Cooper, it is not inconceivable the next time they meet will be in the Premier League. Only Fulham can better Forest’s Championship form since the former Swansea manager took charge in September and, after beating Arsenal in the previous round, this made it five wins in six games. Cooper has challenged his players to prove they have “that winning mentality” when they go to Blackburn on Wednesday. Brendan Rodgers has talked about changing his squad around this summer but Leicester, without an away win in seven games, need to do something about reviving this season before this rot goes any deeper. They were not just beaten here; they were humiliated. The Leicester manager admitted that not even bringing the trophy itself into their team meeting had sufficed to motivate his players. “I said to the players after, that for the first time since I’ve been here I’ve been embarrassed,” Rodgers said. “It was an embarrassing performance. I feel for the supporters. “Forget about the players who are missing. There are players here that may have achieved everything that they can. It’s something we’ll have to look at between now and the end of the season. Until then, they’ve got to have a look at themselves in the mirror and fight like hell to prove they’re good enough to be here. “ They won the FA Cup, and that might be it. But it’s not for me and not what I want to achieve here and do here. Too many players think they’re top players but they’re a long way off it.” The pivotal spell of three goals in nine minutes simply took the breath away as Leicester were blown away. When Forest have been good this season they have been very good – and when Leicester are bad they have been very bad – but this was something else. Keinan Davis had already volleyed, left-footed, against the angle of post and bar when they took flight. Leicester were undone easily for the opening goal. After good work from Djed Spence down the right wing, there was too much space for Brennan Johnson to cross to the back post where Davis outmuscled James Justin to nod the ball down for Zinckernagel to sidefoot the ball home. The BBC were still showing replays of that goal when Johnson, the subject of a £20m bid from Brentford last month, made it 2-0 after Daniel Amartey played a blind back pass towards goalkeeper Danny Ward. Ryan Yates should have headed in and Zinckernagel had a shot deflected wide as Forest smelled blood; some of the first-time pass-and-move from Cooper’s team was exhilarating, not least in the move that led to the corner for the third goal. James Garner’s flag-kick was headed in by captain Worrall, back from injury, who celebrated injudiciously in front of the 4,000 Leicester fans. Quite why the eccentric Brice Samba needed to come sprinting from his goal to cover James Maddison’s channel ball only he will know but Kelechi Iheanacho’s neat finish from a difficult angle did not prompt a recovery. Spence, superb all afternoon, cut inside from wide on the right and kept running after playing the ball in to Zinckernagel whose return pass cut out the creaking Leicester back-line. Spence took one touch to compose himself before sliding his shot into the far bottom corner. It was a fitting finish for a display balancing fight and finesse. Cooper cuts a phlegmatic figure but his quiet satisfaction spoke of his ambition to extend this impressive run. “If we want to be a proper team with a winning mentality, we have to prepare for the next game,” the Forest manager said. “I thought we were good for the result and the performance matched it. First half, some of our attacking play was excellent – dare I say it, it could have been more. “What I liked about it was though we conceded, we didn’t let it affect our focus or mentality. The lads were brilliant.”