The decision comes as JWT defends Unilever's global £140 million Omo laundry detergent account, known as Persil in the UK.
JWT said Sinclair was not involved in the pitch, which is being headed by Craig Davis, the worldwide chief creative officer, and Nick Bell, JWT London's executive creative director.
Sinclair arrived at JWT in March after a stint at the now- defunct US independent agency See. He has spent the past six months working across the Unilever portfolio, which includes Knorr, Chicken Tonight and Lux.
As the creative director at See, Sinclair was responsible for the $150 million Electronic Arts (EA Games) account. However, the account moved to Wieden & Kennedy in January. The agency closed down shortly afterwards.
Sinclair was at JWT's sister network O&M, running creative on Ford in 2003. He joined from Lowe London, where he partnered Ed Morris, now Lowe's executive creative director. The pair created "lost dog" for Volkswagen at DDB London, and "double life" for Sony PlayStation at TBWA\London. They also worked at Bartle Bogle Hegarty.
JWT is not looking to replace Sinclair at present.
The agency is competing against Bartle Bogle Hegarty and Lowe for Unilever's flagship laundry detergent account. A result is not expected until the end of the year.