‘Bloody rubbish’ - Huddersfield Town boss' brutal verdict on Portsmouth flop
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But the Terriers head coach believes he played a key role in revitalising the winger’s career, which helped his side escape Championship relegation.
Following his deadline day arrival at Fratton Park, Koroma would go on to score five goals and provide three assists in 24 outings at PO4.
The then managerless Blues opted to cancel the ex-Leyton Orient man’s season-long loan six months early and returned to Huddersfield.
Yet, under Warnock, who was appointed interim boss until the end of the campaign at the John Smith’s Stadium in February, he would enjoy a change of fortunes.
This saw him play a key role in the Terriers’ Championship survival, where he scored four goals in 16 league appearances.
Despite being warned about Koroma following his failed Pompey loan, Warnock believes he played a key role in the winger’s turnaround and successful end to the season.
He told The Yorkshire Post: ‘I was a big manager for Josh because apparently, before I came, he was bloody rubbish, so it goes both ways!
‘I don't think you play for teams. You play for managers and I think Josh showed what he could do last year.
‘If Josh played for me next year, I would be very disappointed if he didn't get double figures. Very disappointed.
‘I would be giving him a challenge of more than that and Danny is the same. I think both can contribute and they have seen how we can play.’
Koroma remains in negotiations with Huddersfield, along with striker Danny Ward, over extending their stay at the Yorkshire outfit.
After Warnock penned a year-long deal with the Terriers earlier in the month, the 74-year-old is keen for the duo to follow suit.
He added: ‘We have spoken to them. They know how important they are to me.
‘Hopefully in the next few days, we can have decisions on one or two lads like that. We want both of them to stay.
‘I think both of them would want to stay but it is not just me talking. They have advisors and advisors’ advice, sometimes right and sometimes wrong.
‘Hopefully we have some good news in the next few days about those players.’