Barry Ferguson, the midfielder's uncle, reckons the player has served his “apprenticeship” and that it's “time for him to go”.
The Dons star has been linked with Rangers but a concrete offer has arrived from Watford, who won promotion to the Premier League.
A statement from the Pittodrie side suggested they will be doing everything they can to hold on to the 21-year-old.
It read: “The club holds Lewis in the highest regard. Having made a significant investment to both acquire and develop Lewis, he has a bright future ahead of him as an integral part of Stephen Glass’s squad.”
However, Rangers legend Ferguson believes a transfer is in the best interests of the player who has played more than 120 times for Aberdeen.
He won the Scottish Football Writers’ Association Young Player of the Year last season.
"My honest opinion, it's time that Lewis moved on to the next level,” Ferguson told Go Radio Football Show.
"I think he's done his apprenticeship. He did his time at Hamilton, got his move, and he's been a regular at Aberdeen for three seasons, at a good level.
"I think it's time for him to go to the next level and kick on again."
There have also been reports of interest from England in Ferguson’s Aberdeen colleague Ross McCrorie who impressed following his move from Rangers.