Ritchie de Laet was left to nurse Pompey heartbreak as debut delight turned to disaster.
The Blues were heading for an impressive victory over a Nottingham Forest side previously undefeated in 31 home league games after Kanu's 26th-minute strike.
But two goals in the final three minutes cruelly turned the match on its head and presented the three points to Billy Davies' men.
For the impressive de Laet, it was agony after being poised to mark his Pompey bow with a win.
The Manchester United loanee produced an eye-catching maiden outing at right-back just 48 hours after joining the club, defending and attacking in style.
In the end, even that wasn't enough to protect their narrow leading entering the 87th minute.
And De Laet was left to mourn what could have been.
He said: 'I thought we get three points and it would be a perfect start for me.
'We got the goal and knew they would put pressure on us in the second half.
'We just needed to head and kick everything away.
'We did that until just a few minutes before the end and then conceded an unlucky goal.
'Even then we wanted to come away with a point and it would be a good point.
'But in the end there's a dink into the box and he gets the goal – it's heartbreaking for us and for the fans.
'We looked brilliant and, until the final few minutes, you couldn't see them scoring a goal.
'They hadn't really had any shots before then.
'We should have come away with three points but we didn't.
'Now we've got two home games coming up.
'If we win those then we're back up there and you don't know what will happen.'
Steve Cotterill has already praised de Laet's versatility.
The 22-year-old certainly looked accomplished at right-back, with Joel Ward instead asked to slot into the centre of midfield.
With Greg Halford back from suspension next week against Leeds, de Laet could find himself on the move again.
Not that the Belgian international minds as he continues to settle well into life with his new team-mates at Fratton Park.
And he insisted he is prepared to play anywhere for Cotterill during his loan spell until the end of the season.
He added: 'I'll play wherever the gaffer puts me.
'I'm happy at right-back, left-back, centre-back – it doesn't really matter for me.
'The lads are brilliant. It's not a big squad but everyone's joking and laughing in training and I felt at home straight away.
'I know a couple of players from my time at Stoke, which makes things easier for me as well.'