The left-back was sold by United last year to the Portuguese giants and his progress has sparked interest from across Europe - including from Real Madrid and Barcelona.
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Ten Hag revealed on Friday: "We are monitoring the progress of Álvaro Carreras at Benfica, we have a buy-back clause. We loaned him to Preston, to Granada, he didn't play much there... and we had the opportunity to sell him.
"But we have a buy-back clause that allows us to control the situation. We are the ones who control the situation. We have to evaluate the situation, to see if it can be an option for us, we have a specific clause.
"But playing at Manchester United is not that easy, it takes time. You have to develop."