The Republic of Ireland Under-21 striker is closing in on a move to Fratton Park and his first half hat-trick against Shelbourne in last night's EA Sports Cup win could've potentially been his Brandywell send-off.
Shiels revealed after the match it's Curtis' decision to pursue the move to Fratton Park, but the City boss believes his team are much better suited to the Donegal man.
'Well the boy wants to go,' said Shiels, who previously stated the 23-year-old would not be joining the Blues.
'It's just unfortunate he's not going up a level. When they go we want them to go up a level.
'We're better than Portsmouth to be brutally frank. We're a better football team.
'We play a better brand of football so it will be a little bit difficult for him to go there because Division One football in England is very direct.
'It's very simplistic and not as intricate as the way we play.'
Shiels, who is hoping to secure an arrangement with Pompey boss Kenny Jackett which will ensure Curtis remains with the Candy Stripes until the end of their Europa League campaign in the summer, knows it will be difficult to replace the striker who has netted seven times this season from a wide position.
And while he believes Curtis would be better mulling over his options with a couple of clubs understood to be interested, Shiels understand the 2008 FA Cup winning team will offer him a better platform to move up the grades in England.
'It's probably a better platform for Ronan to get up to a higher level,' he conceded.
'The style of play wouldn't be anywhere near where ours is but in terms of potential platform to move further up, it's got bigger potential at Portsmouth than at Derry City because of its location in England.
'But they're certainly not as a good a team as we are.'