So I came to a club halfway through a season after they sacked their manager, one of the players was concerned so I told him that the team's league position will improve and that was promptly placed as a "manager promise"
6 months later, I led the club to the league title and the players went holidaying but this one player I talked to is still speaking in past tense. Basically the promise is still there and his "general happiness" is in the title of this post
How long till he gets it out of his system, he's not even a first-teamer
sometimes player concerns/unhappiness can affect a player for too long. i had a player unhappy for 4 years because i fined him for a red card.
can you talk to him? try to say something that will cheer him up and build a better working relationship with you