All three men have returned to their clubs for further assessment, though Liverpool confirmed that Mignolet is expected to be available for Saturday’s match against Tottenham.
Fellaini was included in Roberto Martinez’s squad on the back of his first start of the season for United in last weekend’s 2-0 away win over Burnley in the Premier League.
Benteke has started all of Crystal Palace’s opening four league matches but is yet to score, and has found the net just twice in 20 appearances in 2018.
The exact nature of the injuries was not disclosed by the Belgian Football Association, who also declined to clarify whether or not they will call up replacements.
According to reports in Belgium, Benteke is suffering from a minor knee issue, while Mignolet is dealing with a hand problem.
Belgium travel to Glasgow to take on Scotland in a friendly at Hampden Park on Friday, before travelling to Reykjavik for their Uefa Nations League opener against Iceland.
The games will be Martinez’s first opportunity to work with his players since July, when Belgium wrapped up an impressive World Cup campaign in Russia with a convincing 2-0 victory over England in Saint Petersburg to secure a third-place finish.