Sunderland will pay an initial fee of around €31.5m for Diarra, a 21-year-old Senegal international who scored in his nation’s win over England at the City Ground earlier this summer. Diarra will arrive as a replacement for Jobe Bellingham, who left for Borussia Dortmund this month.
And Diarra, who will sign a five-year deal, is set to be joined on Wearside by Mandava, who is available on a free transfer when his contract with Atletico expires this week. Mandava has reportedly agreed a two-year contract at the Stadium of Light.
The 31-year-old left-back will provide crucial competition for Dennis Cirkin, who Sunderland hope to tie down to a new long-term contract this summer. Cirkin will this week move into the final 12 months of his current deal and is of interest to his former club Tottenham. Sunderland are determined to keep the England Under-21 international, who was a key figure in last season's promotion under Regis Le Bris.
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Le Bris, however, wants to ensure he has depth throughout his squad, and left-back was identified as one area that needed addressing. Crystal Palace are also said to have held talks with Mandava, while Leeds United - who also wanted Diarra - were also linked, but Sunderland moved quickly and are now finalising the signing of the 49-time Mozambique international.
Mandava, as was the case with Diarra, is expected to undergo his medical overseas.
Diarra and Mandava will follow Enzo Le Fee through the door in what is set to be an extremely busy summer for Sunderland, but it's unclear at this stage whether the Black Cats will win the battle to sign in-demand goalkeeper Nice's Marcin Bulka.
Sunderland looked to be favourites to land the 25-year-old at the end of last week, but strong interest has emerged from Saudi in recent days and various reports in France say Nice have now agreed a deal with Neom SC.
Bulka is entering the final 12 months of his contract and with no sign of an agreement over an extension being reached, Nice reluctantly accepted the need to cash in this summer.
A whole host of clubs have been linked with the Poland international, including Manchester United and Barcelona.
But Sunderland looked to be in pole position until the Saudi twist, with Florent Ghisolfi, incoming director of football at the Stadium of Light, having previously worked with Bulka at Nice.
Strengthening in attack remains a priority for Sunderland this summer, with Ukrainian forward Vladyslav Vanat a leading target. Reports in Spain also claim Sunderland are interested in Almería striker Luis Javier Suárez. However, the 27-year-old, who has scored 33 goals across the last two La Liga campaigns, is said to have his eyes on Champions League football.