After spending time on loan with Plymouth last season, Whittaker returned to the club on a permanent basis in the summer, completing a move from Swansea for a reported £1m plus add-ons.
Whittaker has proven to be one of the standout players for the Pilgrims, having scored 16 goals and provided seven assists in 31 competitive appearances this season.
With 15 of those goals taking place in the Championship, he is currently the league's second-highest scorer and just one strike away from the tally of Blackburn Rovers attacker Sammie Szmodics.
Although Whittaker may have a contract until June 2027, Plymouth have already had to fend off a bid from Serie A giants Lazio.
The 23-year-old is also attracting interest from Brentford and Fulham, who sent scouts to watch him in action in Saturday's 1-1 draw with Leeds United in the FA Cup.
According to Football Insider, the two Premier League clubs will have to meet Plymouth's £15m asking price if they want to sign Whittaker before Thursday's transfer deadline.
The report claims that the two London sides could make a late bid to prise Whittaker away from home park in the closing stages of the winter window.
Despite setting out their financial demands, Plymouth are still reluctant to part ways with one of their prized assets in the middle of the season.
While it remains to be seen whether Brentford pursue a deal, Thomas Frank's side have already experienced a relatively busy transfer window.
The Bees have recruited Yunus Emre Konak, Sergio Reguilon and Hakon Valdimarsson, while they looked set to sign Club Brugge winger Antonio Nusa before an issue with the medical placed put the transfer in doubt.
In contrast to their West London neighbours, Fulham are yet to delve into the transfer market this month, but they continue to be linked with a move for Chelsea forward Armando Broja.