The Wigan Athletic midfielder has been ruled out of this summer's European Championships with an ankle injury
On Friday, Wigan Athletic forward McManaman was ruled out of Stuart Pearce's squad with an ankle injury, which has allowed Delfouneso a place in the 23-man party.
The 22-year-old, who spent this season on loan at Championship side Blackpool, scored in England's Under-21s 3-0 win over Romania in March.
Pearce's men face Italy in their first Group A match of the European Championships on June 5.
"Nathan was very unfortunate not to make the squad in the first place," Pearce told the Football Association's official website.
"He has contributed a great deal to the Under-21s in qualification and he was unlucky that Callum McManaman's form was such that he had forced his way into the reckoning late on."