Striker Benni McCarthy has tried to kickstart his Premiership transfer by threatening to reject future offers if Porto refuse to sell him to Blackburn.
Blackburn and Bolton have been chasing the 27-year-old but his agent Rob Moore has warned that time is running out.
Moore told 24 Horas newspaper: "He'll carry on to the end of his contract if he is not transferred this summer.
"If there are no developments with Blackburn this week that window of opportunity will permanently close."
The South African forward has two years left to run on his contract at Porto and could see them out rather than allowing the club to cash in on his sale.
McCarthy spoke last week about his desire to leave Portugal, where he is beginning to feel unsettled.
"I'm tired of so much ingratitude and falsehood," McCarthy told Portugese sports daily A Bola.
"I deserve to live in peace. It would be best if I go elsewhere to seek the happiness Porto can no longer give me.
McCarthy almost joined Everton in last season's January transfer window and has repeatedly talked of his dream of playing in the Premiership.
"In South Africa, when I was a child, I only watched English football. It is an obsession that I have," he said.