GLASGOW, SCOTLAND - OCTOBER 13: Charlie Mulgrew of Scotland is seen ahead of  the UEFA Euro 2020 qualifier between Scotland and San Marino at Hampden Park on October 13, 2019 in Glasgow, Scotland. (Photo by Ian MacNicol/Getty Images)GLASGOW, SCOTLAND - OCTOBER 13: Charlie Mulgrew of Scotland is seen ahead of  the UEFA Euro 2020 qualifier between Scotland and San Marino at Hampden Park on October 13, 2019 in Glasgow, Scotland. (Photo by Ian MacNicol/Getty Images)
GLASGOW, SCOTLAND - OCTOBER 13: Charlie Mulgrew of Scotland is seen ahead of the UEFA Euro 2020 qualifier between Scotland and San Marino at Hampden Park on October 13, 2019 in Glasgow, Scotland. (Photo by Ian MacNicol/Getty Images)

Revealed: The NINE free agent defenders that Sunderland could target this summer - including ex-Newcastle and Celtic players

The summer transfer window is now officially open.

And Sunderland have much work to do following the club’s retained list.

It was announced that Max Power, Grant Leadbitter, Josh Scowen and Callum McFadzean amongst others were set to leave the Stadium of Light.

Denver Hume and Luke O’Nien are weighing up contract offers whilst loanee Dion Sanderson has returned to Wolves.

That means Sunderland will likely look to strengthen in defence.

Here, we take a look at the free agent options available to Lee Johnson.