The pair are yet to agree new deals despite long-standing discussions with the club, with Hazard insisting in January that he will put to pen to paper once his goalkeeper does.
Real Madrid have long been linked with both Hazard and Courtois, the latter recently claiming his "heart is in Madrid" after a highly successful loan spell with rivals Atletico earlier in his career.
Courtois told Standard Sport last week that he intends on staying at Stamford Bridge for at least one more season, but former Blues defender Gallas says a move to the Spanish giants may be unavoidable.
"To be honest, you have got so many great players in the world that if Hazard or Courtois has to leave, you can find a good goalkeeper to replace Courtois," he told Goal.
"You have good wingers who can replace Eden Hazard. That’s the job of the scouts to find the right players.
"Sometimes you have to accept your best players have to leave or want to leave. Maybe because they want to play in a different league. Maybe because they have already won the FA Cup, Premier League and Carabao Cup.
"I don’t think they have won the Champions League with Chelsea, so maybe that’s why they want to leave and go to another club. You have to ask the question which team they supported when they were kids.
"So many kids will say: ‘I used to support Real Madrid’ or ‘I used to support Barcelona’. So maybe you have to ask that question to Eden Hazard? His dream was to go and play for Real Madrid and maybe you have to accept it.
"You have to accept it. You don’t have to be upset because he has played for Chelsea for many years. All the time he was playing with that blue shirt and giving everything for that club."