The striker marked his Pompey comeback with a 16-minute cameo for the reserves on Monday night.
It represented Main’s first Blues appearance for more than five months following frustrating injury problems.
During the 24-year-old’s absence, Pompey’s medical staff were largely unable to pinpoint the exact source of his discomfort.
However, Main has now been back in training for more than two weeks, initially linking up with the youth team.
His entrance for Conor Chaplin in the reserves’ 2-0 victory over Colchester under-23s was a sizeable step forward.
Certainly Main’s re-emergence is timely, with Eoin Doyle ruled out for the season and Noel Hunt battling to feature during the remaining six fixtures.
But Harris, who oversees the reserves as well as serving as under-18s coach, recognises the need for Main to be allowed time to feel his way back.
He said: ‘It was good to see Curtis back out on the pitch the other night.
‘He’s had a difficult few months and it should give him a lot of confidence coming through that after such a long lay-off.
‘Curtis has been training with us in the Academy and only just got back into it after obviously been having a lot of rehab.
‘He’s working through it and has a long way to go to get to full match fitness, but it is a step in the right direction.
‘When you have been out for that long your first minutes back are vitally important, but it all takes time.’
Main has played 14 times for Pompey, scoring five goals.