The winger was the Blundell Park matchwinner on Saturday with a stunning free-kick four minutes from time.
Naismith had been on the pitch for 10 minutes after being introduced alongside Conor Chaplin in an attempt to eke out a win over Grimsby.
In his last four appearances from the bench, the 24-year-old has now scored twice and laid on an assist against Mansfield.
In addition, there was last month’s magnificent long-range winner against Bristol Rovers in the Checkatrade Trophy, a game he started.
And the in-form Scot remains hopeful of earning a place in a Blues League Two line-up for the first time since October 1 against Doncaster.
He said: ‘Things are going for me at the moment, I am happy with it and confident, as is the whole team.
‘If I got a start next week I would be delighted, or if it was one of the other boys then I am sure they would do a great job.
‘If it was the same team then no doubt they would put in a performance like that again and we will more than likely get a positive result.
‘I am just delighted to be part of it and went on and scored a goal to win the game.
‘I feel in a good place at the moment within the team, I feel a part of it and am happy with the way things are going.
‘Football is so up and down and sometimes you can be out of it, then a week later on top of the world and two weeks later not part of it again, that just happens.
‘It was massive to bounce back on Saturday. Waiting two weeks for the next game was horrible, it wasn’t all smiles and fun in training and that’s what it is all about, you go through hard times.
‘But I am in a good place, as is the whole squad.’
It was Naismith’s fourth goal in a campaign which has signified his Pompey resurgence.
Transfer listed in the summer, the former Accrington man has fought his way back into Paul Cook’s plans.
Such is his blossoming confidence, he took responsibility for the 25-yard free-kick following a foul on Carl Baker.
And the outcome was a sixth-successive away game unbeaten.
He added: ‘You hit 20 free-kicks in training and sometimes walk away because you are not striking them right.
‘Thankfully I hit that in the right spot, as soon as it went over the wall I knew it had a chance, as it turned out.
‘We had good players on the pitch at the time. Conor can hit free-kicks, Danny Rose is very good at them, Gareth Evans fancied himself, but I wanted it.
‘What got me the chance was scoring at Coventry earlier in the season, I have been kind of living off that a little bit!
‘The boys have seen that and thought “We’ll let him have it”.’