Tom Shaw says City will continue to learn as they adapt to life in the Sky Bet Championship.
The Imps made a bright start in front of a crowd of more than 10,500 in their first home game of the campaign, with Ben House going close to opening the scoring before Pompey struck twice to take control of the contest.
Reeco Hackett’s penalty gave City a route back into the game before the break, but Portsmouth added a third in the second half to secure the points.
Shaw felt there was plenty to like about City’s opening-half performance, but admitted his side must become more ruthless at both ends of the pitch as they adapt to the demands of the Championship.
He said: “I thought the first half was a pretty dominant half of football. We were looking at their goal for the majority of the half.
“We’ll go away and dissect the two goals. We can’t concede as many goals as we have in the past couple of games because it makes it very difficult. We’ve got to go away, think about it, and figure out how we can be a little bit more solid.”
Shaw believes the opening weeks of the campaign have underlined the importance of concentration at Championship level, with mistakes punished more regularly by the increased quality of opposition.
He said: “The physical output of our lads is sensational. It has been for a period of time now.
“The bit we’re definitely seeing early doors in this campaign is that mentally we’ve got to be dialled in all the time. We’ve got to be ready and do our work really early because the quality is a significant increase on what we were up against last year.
“Physically, the heart and honesty of the lads is never in question. Mentally, we’ve just got to be a little bit sharper, and we’ve also got to help the players with that by giving them the right framework and structure.”
The occasion marked City’s long-awaited return to Championship football at the ProAmpac Stadium, with Shaw praising the backing his players received.
He said: “People have waited a very, very long time to have a Championship game in this arena.
“The feel of the place was terrific as the lads came out, and then I thought the start made it go another level. The noise was brilliant and there was belief in the arena.
“We’re going to have to be a little bit better than we were to win at this level. That’s the immediate reaction from us as a staff.
“We’ll go away, think about that, obsess over it, find our way, find our rhythm and be a problem for anybody that turns up here against us.”
City’s attention now switches to Tuesday night’s Carabao Cup tie at Blackpool. Shaw added: “We want to win, to stay in that competition and progress. That is the first priority.
“We’ll get the mix right with the priority being to win, but some boys need some minutes. The lads who are waiting and are ready will get that.”