Former Liverpool midfielder Murphy and ex-Everton striker Johnson are out of contract at the end of the season, while USA international Dempsey has just one year remaining on his current deal.
While Jol has claimed that the club are willing to let go of a number of players, the 56-year-old suggested that the club's veterans could stay.
"If they are mentally strong they will stay with us," said Jol. "If not, if they need to play or go somewhere else, we can go on with our transformation."
Fulham failed to cause an upset in the Premier League title race after getting beaten 1-0 by Manchester United last night.