The 24-year-old's future has been the subject of speculation this summer, with Arsenal, Manchester United and Newcastle United all believed to be keen on his services.
However, Villa appear to have recently emerged as the favourites for the Frenchman, and according to RMC Sport, an agreement has now been reached with Leverkusen for his signature.
The report claims that Unai Emery's side have agreed to pay €50m (£43.3m) including bonuses for the France international, who is seemingly preparing to make the move to English football.
Diaby has scored 49 goals and registered 48 assists in 172 appearances for Leverkusen, including 31 goals and 38 assists in 125 outings in all competitions.
Meanwhile, according to Football Insider, Newcastle are preparing to make a new and improved offer for Southampton defender Tino Livramento.
The Magpies are thought to have failed with a £15m bid for the defender, who is widely expected to leave the Saints following their relegation to the second tier of English football.
However, the report claims that Newcastle are now planning to make a bid of £25m in the hope of securing a deal for the 20-year-old during the summer market.
Livramento made the move to Southampton in 2021 following 11 years at Chelsea, and he has featured on 34 occasions for the Saints.
The defender only made two appearances during the 2022-23 campaign, though, due to a long-term knee injury that he suffered in April 2022.
Meanwhile, according to The Athletic, Fulham are keen to bring Ajax defender Calvin Bassey during this summer's transfer window.
The report claims that Marco Silva's side have placed a £15m offer on the table for the 23-year-old, with Ajax yet to submit a response.
Bassey made the move to his current club from Rangers last summer, and he featured on 39 occasions during the 2022-23 campaign, scoring once and providing five assists.
The Nigeria international has another four years left to run on his contract at Ajax, placing the club in a strong position when it comes to any potential negotiations.
Adam Forshaw's future is also being called into question, with the experienced midfielder allegedly receiving a big-money contract offer from Saudi Arabian side Al-Akhdoud.
The 31-year-old's contract with Leeds United expired at the end of June, but the Whites are currently in discussions with him over a potential return.
According to the Daily Mail, Forshaw has been offered a substantial contract by Al-Akhdoud, and he is now considering making the move to Saudi Arabia.
The midfielder arrived at Elland Road from Middlesbrough in 2018, and he has represented Leeds on 91 occasions in all competitions, including 13 outings during the 2022-23 campaign.