Declercq has been declared positive for covid-19, said the Belgian formation which announced the composition of its group on Monday.
Sénéchal, crowned French champion on Sunday in Cholet, had expressed his disappointment at not being able to run the Tour which will pass through his northern region.
The 28-year-old Frenchman will race the Tour for the third time.
But it will be the first time in the colors of the Belgian team of Patrick Lefevere which he joined in 2018.
The Quick-Step team at the Tour de France (1st to 24th July):
Kasper Asgreen (DEN), Andrea Bagioli (ITA), Mattia Cattaneo (ITA), Mikkel Honore (DEN), Fabio Jakobsen (NED), Yves Lampaert (BEL), Michael Morkov (DEN), Florian Sénéchal (FRA)
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