The Wales international has come through the academy system at the City Ground to establish himself as a key player for the Premier League outfit.
Eight goals and three assists came in an ever-present top-flight campaign in 2022-23, yet his progression under Steve Cooper has led to increased interest in his signature.
Nevertheless, despite Brentford known to be admirers of the 22-year-old, Football Insider, the East Midlands outfit will only cash in on his signature if they receive an offer that meets their demands.
Aston Villa are also said to be monitoring the situation, while the likes of Everton and West Ham United have been suggested as potential destinations.
Johnson currently has three years remaining on his contract courtesy of having only penned fresh terms as recently as 2022.
Villa, who are currently preparing to participate in the Europa Conference League playoffs as well as the Premier League, have been linked with Charles De Ketelaere, an alternative to Johnson.
However, it is reported by Calciomercato that Marseille are ready to rival Villa and other clubs in the race for the versatile AC Milan attacker.
The Italian giants have signed a number of offensive players this summer with De Ketelaere having failed to make a positive impact during his first year at San Siro.
As a result, the Belgium international finds himself down the pecking order under Stefano Pioli with the Champions League semi-finalists seemingly ready to cash in on his signature or consider a loan with an option to buy.
Marseille have already enjoyed a productive summer in the transfer market, acquiring the services of left-back Renan Lodi, midfielder Geoffrey Kondogbia and forward dup Iliman Ndiaye and Pierre-Emerick Aubameyang.
That said, it appears that new head coach Marcelino Garcia Toral is eager to further strengthen his squad with Marseille being tasked with trying to make the Champions League through the qualifying rounds.
Staying on the theme of Belgian players and Luton Town are still attempting to sign Blackburn Rovers goalkeeper Thomas Kaminski.
Despite their promotion to the Premier League, the Hatters are without a preferred first-choice stopper having failed to take up the option of signing Nottingham Forest loanee Ethan Horvath on a permanent deal and missing out on other targets.
An interest remains in Kaminski, who lost his place in the Blackburn XI during 2022-23 due to injury and the form of deputy Aynsley Pears.
According to Lancs Live, the Championship outfit are prepared to stand firm over their valuation of the 30-year-old, which is said to be at least £3m.
The report says that Luton have currently only offered in the region of £2m, and an improvement on that bid will be required if negotiations are to continue.