Johnson is out of contract at Craven Cottage in the summer, but Fulham are refusing to give up on their efforts to tie him down to a new deal.
However, interest is mounting in the striker, with QPR and Blackburn both making approaches, and Jol accepts the 30-year-old could leave if a suitable offer is made.
Jol said: "If you get a big offer, you should get a very good player and I think Andy Johnson is a very good player. Anything else is almost impossible.
"Andy is an important player for me. It's important to keep your players - particularly if you can't get other ones in. We'll have to wait and see."