The 25-year-old has made just eight appearances for the Baggies this season and his manager insists he must show a better attitude to break into the team
Sinclair, who is on a season-long loan from Manchester City, has only featured eight times in the Premier League for West Brom since joining in August and was not part of the matchday squad against Cardiff City on Saturday.
Mel revealed that the 25-year-old had not done enough to impress him prior to the game and therefore had not earned a place in the 18-man squad.
The winger has started just four league games for West Brom, but none have come under Mel and the Spaniard expects to see more from him if Sinclair wants to feature in the final seven games of the campaign.
"He [Sinclair] didn't play simply because the players who are here were the best for this game," Mel told reporters.
"I can only pick 18 players for the squad. Only 11 can play.
"I choose the best players. We need players with a positive attitude and a strong attitude right now."
Sinclair's last appearance under Mel came as a 79th-minute substitute in the 2-0 defeat to Hull City earlier this month.