Joe Gallen was the Millwall assistant manager at the time and he remembers it as an “us or them” situation. Millwall were hovering above the Championship’s relegation places and Portsmouth were embedded in them, having been docked 10 points for their descent into administration. It was 10 April 2012 and Millwall’s visit to Fratton Park was afoot.

It would be one of those dogfights, played out in front of a fraught crowd of more than 15,000 and one player would make the difference. He did so by scoring the only goal, with what has since become a trademark finish – an unerring low shot from the edge of the penalty area. It was the night when Harry Kane gilded his Millwall hero status.

The striker was 18 and he had joined on loan from Tottenham Hotspur for the second half of the season. In Millwall’s previous match, he had scored the opening goal in the 2-0 home win over Hull City and, in the one after, he would again score the first goal – in the 2-1 victory against Leicester City, also at the Den.

It was Kane who drove Millwall’s end-of-season revival, in which they won five games in a row – beginning against Hull – and drew the last one, at home to Blackpool, Kane scoring a late equaliser. They would finish in 16th place, 17 points clear of third-bottom Portsmouth and, in the end, it looked comfortable enough. However, the final table failed to tell the story of a season of struggle and pressure of the most exacting nature.

“My loan at Millwall was a big part of my development,” Kane said as he prepared for Tottenham’s FA Cup quarter-final against Millwall at White Hart Lane on Sunday. “I was 18, we were in a relegation battle and it turned me into a man. I played in difficult, high-pressure games and I managed to come out of it positively. I had a great time at the club and it will be interesting to play them again. A lot has changed since I left but I’m looking forward to it.”

It was not easy, at first, for Kane. Although he scored twice in Millwall’s FA Cup replay win over Dagenham & Redbridge, he drew blanks in his first eight Championship appearances. His third game was the 6-0 home defeat by Birmingham City. But Gallen remembers how it clicked for Kane. The striker’s first goal in the league came in the 3-1 win at Burnley in late February 2012 and it was the prompt for him to finish the season with seven in 14 matches.

“It’s safe to say that we probably would not have stayed up had Harry not come,” Gallen said. “It was a gamble bringing him because of his age and it being Millwall and the Championship, which is such a tough league. But he just went on a great scoring run. His goals and the way he played changed it all for us.”

The deal came about because of the connection between Kenny Jackett, the Millwall manager at the time, and Tim Sherwood, who was then in charge of the Tottenham youth set-up. The pair have been friends since their playing days at Watford. Ryan Mason moved on loan from Tottenham at the same time but, because of ankle problems, the midfielder – who is now at Hull – could not string a run of games together.

It is stating the obvious to say that Millwall is a difficult place to play. Kane had made his debut for Tottenham at the start of that season – he played six matches in the Europa League under Harry Redknapp, scoring once against Shamrock Rovers – and, in the words of Gallen: “People were like: ‘Who is this kid from Tottenham?’” Kane won them over with his single-mindedness, as well as his quality.

“The Millwall fans can be tough,” Gallen said. “They can be great – once they are on your side, they can be amazing. Once they’re against you, they tell you how it is. Personally I like that a lot. But with Harry, he was completely unfazed. He just had an inner belief.

It took him a game or two and then it just clicked. Goals breed confidence for everyone and once they started to come for him, he was away. There was no stopping him, really. I worked more with the forwards but I didn’t need to talk to Harry too much – at half-time or before games. With some players, you are constantly saying: ‘Make this run or hold the ball up.’ With Harry, there was no need.

“He was very confident in his own ability but pretty quiet. That said, if things weren’t going his way on the training ground, in terms of him not getting the ball, he would speak up – in a nice way. There’s no baggage with him. He was very low maintenance. He didn’t give it: ‘I’m a Tottenham player.’ Both him and Ryan Mason were really decent young lads. We’ve had some lively ones over the years but they definitely weren’t like that off the pitch.”

Gallen returns on several occasions to the same subject: practice. It was all Kane ever wanted to do. The Tottenham manager, Mauricio Pochettino, talked last week about how Kane could get angry when he was ordered off the training pitch and it struck a chord with Gallen.

“In my 20 years of coaching, I’ve never seen a player practise as much as Harry,” Gallen said. “He was always practising from the edge of the box. That was his thing. He’d literally have to be chucked off the training pitch.

“Kenny [Jackett] would be watching from his office and he’d say: ‘Joe, you’ve got to come in. He’s going to pull his quad, here.’ I’d say to Harry: ‘Come on, we’ve got to go. Kenny’s going to kill me.’ And Harry would be annoyed. He would be groaning at me, saying: ‘Come on, let’s do some more.’ He’d be looking at me – not happy. The assistant gets all the abuse.”

In the story of Kane’s career, the game against Portsmouth occupies a vital place. “I’ve seen Harry quoted about it and he said it was a moment when he realised what it all meant,” Gallen said. “The prospect of defeat was terrible and he felt that. It was men’s football, with people playing for their livelihoods. He grew up in that environment.

“Had we lost, we would have been thinking: ‘It’s going to be tight this year.’ But once we won, we were thinking: ‘OK, we can probably do this.’ Harry’s goal was from the edge of the box, as usual. He was deadly from that ‘D’ area. He’s the best I’ve seen from that area. It’s like a six-yard tap-in for him but he would practise so much on it.”

At the end of it all, Kane was named as the Millwall young player of the season. “I’m sure they would have liked to give that to someone from the club, rather than a loan player, but they had no choice,” Gallen says. “It was so much about Harry that it had to be him. He was by far and away the best. The fans absolutely loved him.”

The Guardian Sport