Gylfi Sigurdsson is "irreplaceable" for Swansea City, according to former Swans and Wales striker Ian Walsh.
Everton have agreed a club-record £45m deal to take the Iceland midfielder to Goodison Park from the Liberty Stadium.
The 27-year-old will have a medical in Liverpool on Wednesday.
"To lose that creativity, there isn't a player like that at Swansea at the moment," said Walsh on BBC Radio Wales Sport. "How much does it cost to replace a player like that?"
And Walsh added: "In my personal opinion, I think Gylfi Sigurdsson is irreplaceable."
The Iceland international scored nine goals and contributed 13 assists last season to help Swansea avoid relegation from the Premier League.
His 34 goals are the most scored by any Swansea player in the Premier League, and is also their leading creative influence with 29 assists.
The £45m fee is £5m less than the Swans wanted for Sigurdsson.
They have also shown an interest in striker Wilfried Bony, with last season's top scorer Fernando Llorente recovering from a broken arm and set to miss Saturday home game against Manchester United.
"Swansea have got a lot of hard work now, [Swansea manager] Paul Clement has got to have two or three players in the pipeline ready to go," added Walsh.
"They've got the money now, but they have to use it wisely.
"Joe Allen is talent but his price has gone up about £10m after his performances for Stoke last season.
"They need to strengthen up top because you don't know what Llorente is going to be doing. They need players in two or three different positions."
Former England and Arsenal striker Ian Wright believes Sigurdsson is the best player in the Premier League outside the top five clubs.
Speaking on Sky Sports News, he said the transfer was bad news for the Welsh club.
"It's fantastic for Everton, but a disaster for Swansea City," he said.