And Danny Cowley believes the winger will need to find a single-minded strength of character if he is to hit the goal trail on a consistent basis.
Harness goes to Rotherham this weekend with two goals in the past three games to his name, and a trio of finishes on the board this term.
With Ronan Curtis suspended at the New York Stadium, there will be additional onus on the 25-year-old to bring the goal threat and creativity Pompey require.
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Cowley has absolutely no doubt he can do that - and has laid down a clear target for Harness moving forward.
He said: ‘Marcus has top-end ability, that’s there for everyone to see.
‘He’s almost got to be a bit more greedy, a bit more single-minded in the final moment.
‘He’s always looking to create and assist, but I think he can be a little bit more single-minded.
‘Someone of his quality, you want double figures (goals) really.
‘That will be the definer for him - whether he can find that number of goals consistently.’
Harness has showcased undoubted ability in his two years at Fratton Park, following his £900,000 arrival from Burton Albion.
That is talent which ensures he remains one of Pompey’s most bankable assets, and a player who’s generated interest from suitors.
But it hasn’t always so easy for Harness to bring his talent to the fore consistently, especially under former boss Kenny Jackett.
Cowley believes there’s a psychological element behind that, as the Pompey man places pressure on himself to perform.
He added: ‘The challenge for Marcus is to do it consistently.
‘To be fair to Marcus, he’s trying to do the hardest things on the pitch - create goals and score goals.
‘The consistency can always be a bit less with the high-tariff players.
‘Marcus can sometimes be quite hard on himself, so it’s for him to stay calm and be in the moment so he can always be in a good place psychologically to perform his best.’
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