Cardiff City could lose captain Steven Caulker and midfielder Gary Medel after failing to stay in the Premier League.
BBC Wales understands both players have release clauses in their contracts which could be triggered by the club's relegation to the Championship.
However, potential buyers would have to meet the Bluebirds' valuation.
Defeat at St James' Park saw Cardiff drop back into the second tier after just one season in the Premier League.
Midfielder Medel, who captains Chile, joined Cardiff from Sevilla in August 2013 for a club-record £11m.
His arrival came a week after 22-year-old England international defender Caulker, who cost the Welsh club more than £8m from Tottenham.
Former Wales and Everton captain Kevin Ratcliffe fears Medel, 26, will be particularly difficult for Cardiff to keep.
"He's captain of his country playing in the World Cup in Brazil this summer," said Ratcliffe, a former Bluebirds player.
"Would you want to come back to play in the Championship after that?"