He has matured into the goal-scoring scourge of Pompey’s defence.
Now John Marquis has the opportunity to wreck the Blues’ club record-equalling attempt at Doncaster.
As if the striker initially unleashed upon the Football League by Kenny Jackett didn’t have enough motivation against his former employers.
The 26-year-old has netted four times in his last three appearances against Pompey, where he spent two months on loan during the 2013-14 campaign.
It was a period when Marquis received criticism from some sections of the Fratton faithful, nonetheless he has gone on to enjoy a prolific career at the Keepmoat Stadium.
Certainly Joe Gallen recognises the threat provided by the hard-working striker who emerged through Millwall’s ranks.
Gallen was by Jackett’s side when Marquis broke into the Lions’ first-team as a teenager, making sporadic appearances.
Now the Doncaster goal getter will line up for the opposition as Pompey bid for a fifth-straight league win from the season’s start.
Gallen said: ‘I happen to think John has done brilliantly for Doncaster.
‘He’s been on loads of loans prior to settling. The best thing for him was to get a lot of experience and play plenty of games – and he has found a home at Doncaster and settled.
‘He’s a big player for them, very aggressive, has a lot of energy, doesn’t know when he is beaten, can head it into the net and score a variety of goals.
‘Truth be told, at Millwall he was young and the club was in the Championship. He still played and scored some goals when he was in, but there were other options as well.
‘Sometimes with centre-forwards they need more time to develop and when they get to 25, 26 or 27 there's a maturity, experience and confidence. Things can really gel at that age.
‘For a centre-forward who is aged maybe 21-22, you can see they have the potential but cannot quite put it all together – and I think that's where John was.
‘He definitely always had the potential and is very much Doncaster’s danger man.
‘We have been talking about him and it’s a tall order to keep him quiet for the whole of the game.
‘He will give the defence a good run for their money because he’s very aggressive and will be up for it, for sure.’
Marquis scored twice in six outings for Pompey while Guy Whittingham was boss.