The Tykes are struggling in the bottom three of the Sky Bet Championship and the 22-year-old missed the home defeat at Oakwell against Alex Neil's side.
Reports had earlier claimed Diaby had failed a test ahead of the 3-2 defeat at Blackburn in November and that Struber had gathered his players to tell them ahead of the Preston game.
Asked about the reports after the game, Struber said: "It was distracting. The process is running but it's not the right time to speak about Bambo."
Diaby, who was born in Spain and also has Senegalese nationality, joined Barnsley in 2019 from Belgian side Lokeren.
He signed a four-year contract with Barnsley in July and made the latest of his 17 appearances for the club in the 2-1 home win over fellow Championship strugglers Huddersfield on January 11.