Ian Woan is set to leave Pompey and become assistant manager at Watford.
The Hornets want the Blues’ first-team coach to serve as new boss Sean Dyche’s right-hand man.
Pompey have given permission for them to speak to Woan and a deal is likely to be completed over the next few days.
Woan is best friends with Dyche, who this week was appointed to replace Malky Mackay.
Joining him at Vicarage Road is an opportunity ex-Nottingham Forest midfielder Woan is eager to take.
He arrived at Fratton Park in July 2007 as under-18s coach as part of Paul Hart’s new regime and worked his way up to become first-team coach.
Now the popular 43-year-old will leave Pompey after four years.
Steve Cotterill said: ‘To be fair to Watford, they have done it all the right way and made an approach for him to go and work alongside Sean Dyche.
‘To be honest, when Sean Dyche got the job I knew this one would be coming because they are really close.
‘Woany is his best mate and vice versa and they have ended up being friends and their families are friends.
‘To be fair to Watford, they have approached us in the right manner and asked to speak to him.
‘I see that happening soon.
‘It’s a shame, really, he’s a good part of the group.
‘Him and Guy (Whittingham) signed new contracts last season which showed how highly I thought of him.’
Meanwhile, West Ham have parted company with coaches Paul Groves and Dave Coles.
The duo walked out of Fratton Park last summer to link up with Avram Grant again. But Hammers boss Sam Allardyce has decided not to retain them.