The 29-year-old represented Brazil at the World Cup, but could only captain his side to a disappointing fourth place finish after a 7-1 humiliation at the hands of Germany in the semi-finals and a 3-0 defeat to Netherlands in the third-place play-off.
And the former Auxerre coach believes the ex-AC Milan man's performances have been plummetting in quality since the tail end of the 2013-14 Ligue 1 campaign, though he can offer no explanation for the slump.
"Thiago Silva has been worrying since March," Roux told France Football. "We were told then that he was saving himself for the World Cup.
"After seeing his performances in Brazil, I hope he was actually saving his best for Ligue 1.
"When I see a player at this level commit such errors, whether it be facing Colombia or the Netherlands, my first instinct is to wonder if something serious happened in his life. Because the decline is so drastic that it must be a rational explanation."
The PSG captain will be looking to put the disappointing tournament behind him and turn his attentions back to club duty, where the Parisiens will be aiming to defend their Ligue 1 title this campaign.