The 25-year-old rounded off an excellent team move at The Valley when he latched on to John Marquis’ back heel, before firing home past Craig MacGillivray.
It helped the Blues pick up a 2-2 draw in south London, with Cowley pleased to see the Republic of Ireland ace on the scoresheet again after watching him flourish in a new role behind a front two of Ronan Curtis and John Marquis.
Harness is expected to fulfil a similar brief in a 3-5-2 formation when the Blues head to Burton in League One tonight.
The game will be something of a homecoming for the attacker, having moved to Fratton Park from the Brewers in 2019.
He’ll be keen to impress, having bagged a hat-trick in the same fixture this time last year.
But in order to impress his manager even more, Cowley believes the player must adopt a greedier attitude in front of goal.
The Blues head coach said: ‘Marcus is an intelligent player, he’s really good at finding his own space and has a wonderful ability to speed the play up.
‘He has this lovely penetration off of his dribble and he’s a super finisher.
‘Marcus has such a clean strike of the ball, and we see it day-after-day on the training pitch and sometimes he’s almost so selfless that he’s always trying to create the goal rather than actually taking the chance himself, so it was great to see him score the goal on Saturday.
‘He obviously goes back to Burton on Tuesday which is a club he knows very very well and there's no doubt that he’ll be looking forward to the game.’