Confirmed in an interview with producer Gareth Reeder.
While Konami has already released a playable demo for PES 2017, EA has not yet revealed when FIFA 17's demo will be available, or at least, now formally. Gamereactor recently spoke with Garreth Reeder, FIFA 17's Lead Producer, who told us the demo will be coming out on September 13. No more details were offered, but we would bet on PC, PS4, and Xbox One for the platforms (the PES 2017 demo skipped PC).
As for content, we can only guess, but here's the content in the Gamescom demo:
Real Madrid, Manchester United, Chelsea, Olympique Lyonnais, Paris Saint-Germain, Seattle Sounders, Tigres UANL, FC Bayern Monaco, Gamba Osaka, Inter, Juventus
Stamford Bridge, Suita City Football Stadium, Centuitylink Field
FIFA 17 will be coming to several platforms on September 29. In the meantime, you can check the full interview with Gareth Reeder below.