Paul Warne condemned any Rotherham fans who criticised Ryan Williams on social media after he joined Pompey.
But the Millers boss admitted he expected the winger to join a Championship side rather than a League One rival following his New York Stadium departure.
Williams made his return to Fratton Park last week when he signed a two-year deal.
The Australian turned down fresh terms at the south Yorkshire outfit following their relegation from the second tier last season.
But instead of joining a club who'll compete in the Championship, Williams moves to a Blues side who’ll also be pushing for League One promotion next term.
That led to some backlash from Rotherham supporters.
But Warne told fans of his club not to be “haters" and respect Williams’ decision.
The Rotherham manager told the Sheffield Star: ‘I don't like the thought of our fans criticising any of our players for leaving.
‘The players are here, they get paid to do a job and they give it their best. If they then choose to move on then so be it. You don't have to be “haters”.
‘It's just the way that it is. They're not being offensive to fans personally. He wanted to play in the Championship. You can't blame him for that.
‘It probably didn't work out as well as he wanted and he thought that Portsmouth was his best move.
‘I sincerely wish him luck, apart from when he plays us.’
Warne reckons Championship sides did make enquiries about Williams but the club he ideally he wanted to join didn’t table a deal.
He added: ‘I'm disappointed he's gone. I spoke to Ryan many times last season about staying on.
‘He really wanted to kick-start his international career.
‘When I spoke to him, he thought he would have an opportunity in the Championship. In fairness, I thought he would as well.
‘The player gets his head turned a little bit. That's what's happened. It's that late in the day now that the Championship club he wanted possibly didn't come forward.
‘Then Portsmouth came forward. I can't put words into his mouth. This is just what I think.’