Sinclair had been cut from 40/1 to 1/4 to join Newcastle after a run of bets.
The 24-year-old, who has struggled to make an impact at Manchester City since joining last year, has been subject to a flurry of sudden interest with Sky Bet to move to St James' Park, prompting the Magpies to replace Everton at the head of betting on his next club.
Sinclair made just 11 appearances including two starts in his first season at the Etihad Stadium.
Sky Bet Football Trader Chris Spicer said: "In a market which has previously seen very little money, we have taken a strong run of bets on Sinclair's next club being Newcastle, prompting the price to plummet.
"With nothing apparent in the media domain on such a deal happening, we're assuming the money is coming from those in the know, although it's always difficult to tell during the season of speculation."
However . no other bookmaker has had any "rush of bets" . and it seems nothing more than publicity stunt to promote Sky Bet.
Source: Newcastle United Mad