Nurmagomedov announced the heavyweight main event to ESPN on Wednesday.
Additionally, Hector Lombard will face Thiago Silva on the card's co-main event. The Eagle FC program is expected to take place at FLXcast Arena in Miami. All four combatants in the main and co-main events are former UFC veterans.
Dos Santos (21-9) is considered among the best heavyweights in MMA history. He fought exclusively in the UFC from 2008 to 2020, winning its heavyweight championship by knocking out Cain Velasquez in 2011. The UFC released dos Santos in 2020 on the heels of a four-fight skid.
The 38-year-old had been scheduled to box Kubrat Pulev under the Triad banner in February, but the event was canceled.
De Castro (8-3) broke into the UFC in 2019 on Dana White's Contender Series, but was not re-signed after going 1-3 in his first four fights. Born in Cape Verde, De Castro has won his last two fights, including a first-round submission in his Eagle FC debut.
Lombard (34-10-1) hasn't fought in MMA since 2018 but has remained active in bare knuckle boxing. The 44-year-old has been fighting professionally since 2000, and held a title in Bellator MMA. Silva (21-9), of Brazil, has fought in the UFC, WSOF and most recently KSW in Poland. This will be his first fight since 2019.