The 25-year-old, who is on a season-long loan from Manchester City, has made 11 appearances in all competitions for the Baggies since the summer.
Despite Mel recently insisting that Sinclair has a future at the club, the Spaniard wants the winger to be more positive in training.
"I can only pick 18 players for the squad. Only 11 can play," he is quoted as saying by Goal.com.
"I choose the best players. We need players with a positive attitude and a strong attitude right now."
West Brom are back in action on Saturday when they travel to Norwich City in the Premier League.