AP Source: Man United to sign defender Telles from Porto

Manchester United has agreed to a deal in principle with Porto to sign defender Alex Telles, a person familiar with the move said ahead of the transfer window closing on Monday.

United will pay 15 million euros ($18 million) plus potentially another 2 million euros depending on Telles' success with the club, the person said. They spoke on condition of anonymity because they were not authorized to discuss the transfer ahead of the move being completed.

Telles is due to sign a four-year contract with the option for a further season.

United is signing the left back after a 6-1 home loss to Tottenham in the Premier League on Sunday exposed the team's defensive problems.

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