The 24-year-old has been linked with a host of clubs including Middlesbrough, Bristol City, Crystal Palace and Sheffield United but Lambert already has plans in place should he lose his star man.
The winger almost left the League One outfit last summer only for his proposed move to Crystal Palace to collapse after a shoulder injury caused him to fail the medical.
And Lambert said: "I'd like to hold onto Mark. He's an exciting player but in the end money will decide what happens. Players will have to go because I want to bring others in. There is not the same money around as when this club was in the Championship and I understand that.
"I expect things to happen for Mark, if an offer comes in that is good for the club. He would definitely leave a hole. You never like to see your best players leave, but I'm trying to get a few more players in.
"I know how many I want to bring in - there are six - and I want to be working with a first-team squad of just 21 or 22."