Joachim Low's charges beat Argentina 1-0 in the final of the competition at the Maracana in July to secure a fourth trophy for Die Mannschaft in Brazil.
Mertesacker told The Telegraph: "For me personally, it is difficult at the moment because that was really a long season last year, and to come back from the World Cup and get the motivation back – I am nearly back – but you can feel there is something missing at the moment.
"It's a bit of everything. Such a long season last year, without a winter break, and with a World Cup. That is nearly 100 days ago, so I think it is time to raise the game, raise the mentality, to come back and just think about Arsenal."
Germany have struggled since winning the World Cup, losing to Poland and drawing to the Republic of Ireland in the Euro 2016 qualifiers.