A late own goal from Ibrahima Sonko and a last-gasp winner from Marcus Tudgay broke Pompey fans' hearts as their team to yet another defeat.
This one was extremely difficult to take after Kanu had given Pompey a deserved lead in a game that Pompey were comfortable in until late on in the second half.
Ritchie De Laet made his debut at right back with Joel Ward being pushed into a central midfield role, where he performed fairly well. David Nugent was moved to left wing with Kanu playing behind John Utaka who seemed to relish his role as the lone striker.
The first half was evenly contested to begin with as neither goalkeeper was seriously tested. Pompey looked threatening with De Laet and Lawrence making a good combination on the right with Kanu orchestrating attacks in an advanced midfield role.
And it was the Nigerian who opened the scoring for Pompey with a good goal. Nugent capitalised on a slip from a Forest defender, ran the ball down the pitch and passed to Kanu who rounded Lee Camp and picked his spot to score his first goal of the season.
Pompey were in the ascendency and Camp did well to tip Lawrence's free kick around the post as the ball was destined for the top corner. Forest's only real effort came from Tudgay who "Quashie'ed" his effort from inside the box in an otherwise comfortable half.
The second half began in much the same way with Pompey getting the basics right at the back and Utaka making a couple of decent runs at the Forest defence to keep some pressure on at the other end.
However, Steve Cotterill withdrew Utaka and then Kanu later on in the half and on came Tom Kilbey and Hermann Hreidarsson. Lawrence could have sealed the win for Pompey following a counter-attack but opted to shoot over the bar rather than play in Kilbey.
Forest would then go on to dominate the latter stages of the half as they continued to create chances but it looked like the defence would stay strong until a stroke of bad luck saw Sonko slice the ball into his own net.
That was hard to stomach but most Portsmouth fans were still fairly happy to take a draw from the City Ground, only we didn't even get that after Tudgay's header from Lewis McGugan's long ball looped over Ashdown in the final minute of stoppage time.
It was an absolute sickener and there were rumours that Steve Cotterill had quit the job in a rage after the final whistle. However, the Pompey manager has said he'll keep going.
I've not criticised the manager much this season but I feel that he must shoulder the blame for this defeat. Utaka and Kanu may have had to come off through injury and fatigue respectively but that's no excuse for allowing the team to fall back and invite Forest to pressure us.
That meant that there was no "out ball" and nobody upfield to hold the ball up, meaning that every punt upfield by a Pompey player resulted in the ball coming straight back at them as Forest kept coming forward.
I still have some sympathy for him though as he still doesn't have anywhere near a full squad of players to work with so his hands were tied today with only a goalkeeper, two defenders and an untried midfielder to call upon if needed.
The situation hasn't been helped with Mokoena's tenth booking of the season which now rules him out for the next two games. At least Greg Halford will be back...
If Pompey are not to be dragged into a relegation battle then the board need to work hard to bring in the players that Steve Cotterill wants. If he doesn't get them then he may walk and worse yet, we may end up getting relegated.
Pompey Player Ratings
Jamie Ashdown: 7 - Caught everything and made no mistakes.
Ritchie De Laet: 7 - Impressive debut, got forward well and made some good challenges.
Ibrahima Sonko: 6 - Headed everything away but distribution was awful.
Aaron Mokoena: 7 - Much better than in previous games.
Carl Dickinson: 6 - Also improved.
Liam Lawrence: 6 - Unlucky not to score.
Hayden Mullins: 6 - Busy performance.
Joel Ward: 6 - Slotted in well.
David Nugent: 6 - Worked hard but didn't see much of the ball.
Kanu: 7 - Instrumental in Pompey's success.
John Utaka: 7 - Caused the Forest defence problems.
Substitutes
Darryl Flahavan
Hermann Hreidarsson (Kanu 81): N/A - Brought on to shut up shop.
Ricardo Rocha
Tom Kilbey (Utaka 72): N/A - Never really noticed.