Fellaini was injured in Manchester United’s win at Hull City on Saturday.
"He has an injury in his back. It's not good," United manager Jose Mourinho said after the game.
Fellaini missed Belgium’s first training session under new coach Roberto Martinez on Monday and an announcement on his availability should be made later on Tuesday.
Dendoncker, 21, was the first Belgian-based player picked by Martinez. He won his first cap last year. The friendly against Spain in Brussels on Thursday is followed by a trip to Cyprus next Tuesday.
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(Reporting by Mark Gleeson; Editing by Ed Osmond)
