Their careers depend on performance, therefore they’ll give 100% every game if fit. No professional is bothered by negotiations, clubs are employers and like all relationships, even us mere mortals have with our own employers you don’t want that to be soured as it can impact your next job. That’s why players don’t hold loyalty to clubs as fans do, it’s different, publicly they’ll tow the club line, support the fans, do the PR, smile and greet enthusiastically and then repeat at the next club, almost mercenary, once moved on l doubt they even remember the fans previously. No player who’s leaving at the end of their contract would drop performance knowing their leaving to another club, as that club would still be aware & they wouldn’t want that happing too them. In fact a player maybe even more motivated to perform as it looks better for their careers.