According to Kenny Jackett, the winger is ready to come back from his thigh injury to fire up the below-par Blues.
Harness has been sidelined for the last five matches, an absence coinciding with one point from a possible six in League One.
The summer recruit had been in excellent form before sustaining the problem during Pompey’s 1-1 draw at Blackpool last month.
And, having returned to training at the back end of last week, Jackett expects Harness available for Saturday’s visit of bottom club Bolton.
He said: ‘Marcus is good, he’s coming on really well and will be fit for Saturday.
‘I wasn’t going to risk it for Southampton. He was training at the back end of last week, but the fact it was a muscle injury meant I was going to give him a full week, so he’ll be available for Bolton.
‘It’s great news for us, he's had a bright start and we are very pleased with him.
‘With a muscle injury, and being quite an explosive player, we wanted to make sure when he gets back, he stays back.
‘I am confident Saturday he will be fit and available.’
Meanwhile, the Blues are continuing to assess Oli Hawkin’s foot injury sustained against Southampton.
While Jackett is unable to pinpoint Lee Brown's comeback date.
He added: ‘Lee is a little further off and outside running, but still has a problem with his Achilles and isn’t 100 per cent.
‘It’s difficult to put a timeframe on him.’