The 24-year-old Manchester City winger struggled to produce his best as the Belgians were beaten 2-0 by Italy in their Euro2016 opener in Lyon on Monday night.
After the game, Wilmots admitted the usually dynamic De Bruyne was down on his usual output but felt the team as a whole failed to fire on a night where Emanuele Giaccherini and Graziano Pelle provided the goals for the winning Italians.
Wilmots said of De Bruyne: "Is the kid tired after a long season?
"Probably, but I am not going to slate him, he has been a big player for us in the past and I left him on as he is often able to bang in an assist," said Wilmots.
"There was a breakdown in communication for their opening goal, which can't happen at this level."
Belgium will now look to bounce back when they take on the Republic of Ireland, who were held to a 1-1 draw by Sweden, in Bordeaux on Saturday.