The former Manchester United attacker comes off contract this month and is in Bosman talks with the Potters.
"We are hopeful that Diouf comes," Coates told BBC Radio.
"He is a striker, who Mark Hughes has already wanted last year. Twice we have thought that we had him, but then the deal has failed. As it sometimes happens in football."
For the big stories and the banter follow us on Twitter: @tribalfootball