Leverkusen have already lost their playmaker Florian Wirtz and a defender Jeremie Frimpong to the Premier League champions Liverpool, while center-back Jonathan Tah has transferred to Bayern Munich.
Former Arsenal player Xhaka has been rumored to be moving to Sunderland, with his agent informing German media that the 32-year-old Switzerland captain has a preliminary agreement with the newly-promoted Premier League team.
"The agent can claim anything. However, this club has already lost three significant players," Ten Hag remarked to German media on Tuesday.
"We won't allow any more departures; that's not feasible. It would undermine the team's structure and culture."
"It's obvious — Granit is a leader, he signed here for five years and has three years remaining on his contract. He is too crucial for us to let him leave."
Xhaka played a key role in helping Leverkusen achieve a double in the 2023-24 season, as they secured their first Bundesliga title without a loss, won the German Cup, and reached the Europa League final.
He has made 99 appearances for Leverkusen, netting six goals and providing nine assists.
Former Manchester United manager Ten Hag took over from Xabi Alonso in May after the Spaniard departed to manage his former club Real Madrid."