Aston Villa striker Delfouneso returned to his club following Thursday night's 5-0 thrashing of Iceland, which saw him pick up a hamstring injury.
The 20-year-old was withdrawn after just 11 minutes in Colchester and will not travel to Mons.
Blackburn midfielder Jason Lowe and Tottenham defender Adam Smith, who is on loan at MK Dons, look set to shake off respective hamstring and ankle injuries.
The squad is also expected to be bolstered by the return of Liverpool's Jon Flanagan and Everton's Ross Barkley, who were drafted into the under-19s party for Thursday night's clash with Denmark.
England Under-21s will be looking to maintain their 100% Group Eight record in Belgium on Monday having cruised to four wins out of four.