Diouf, 21, impressed in his debut Premier League season, providing five assists in 32 appearances and catching the eye with his crossing ability, prompting Brentford to explore a move as they look for long-term competition for Rico Henry at left-back.
However, having already raised close to £150m this summer through the sales of Mateus Fernandes and Crysencio Summerville, West Ham United are determined to keep the bulk of their remaining squad together following relegation to the Championship, and it remains unclear whether Brentford will return with an improved offer for the Senegal international.
Henry has struggled for fitness since returning from a second anterior cruciate ligament injury, managing just 25 Premier League appearances last season before a hamstring problem ruled him out of the final nine games, forcing Keane Lewis-Potter to deputise out of position. Lewis-Potter has previously said he does not see his long-term future at left-back, while right-back Aaron Hickey, who can also cover the position, has himself been hampered by injuries, leaving Brentford light on natural options behind Henry.
Elsewhere at the Gtech Community Stadium, Cox has joined Barnet on loan for the season having progressed through spells at Bristol Rovers, Crawley Town and Shrewsbury Town since arriving at Brentford from AFC Wimbledon in 2021. Barnet manager Dean Brennan said the 23-year-old had left a lasting impression during a previous meeting between the sides.
"He was magnificent against us last season when we played against Shrewsbury and he was man of the match in that game," Brennan told the club website.
"It's a positive deal for everyone and it's a big loan for him and his career. He needs to be successful during this loan and I feel like we're getting a hungry goalkeeper with lots of ability, an ex-England youth goalkeeper and he's played a significant amount of EFL games," Brennan added, with Cox set to compete for the number one jersey as Barnet look to build on last season's League Two campaign.