The striker arrived from Liverpool in the summer but has found minutes hard to come by at Vicarage Road, making just one start in the Premier League.
However, he got the nod for Saturday's FA Cup tie against Burton and his 77th-minute strike sealed a 2-0 victory for his side.
Sinclair admits he always knew that joining the Hornets would prove a challenge but is hopeful that his first goal for the club can help persuade Mazzarri that he deserves more of a shot at a run in the first team.
"Hopefully he can afford to put me in in the forthcoming games," he said. "I always knew that coming here was going to be a work in progress, it wasn't a matter of coming here and demanding game time. It was always a work in progress.
"I know as a young player I have things to work on and I am just trying to do that. As long as opportunities like this keep on coming I will do my best to take."