The Northern Ireland international joined the Hornets in the summer of 2014 from Blackpool and he was integral in helping Watford to win promotion to the Premier League under Jokanovic.
Cathcart has fallen down the pecking order at Vicarage Road over recent times, with injuries also keeping him out of contention.
The 28-year-old made just 15 Premier League appearances last term and he has only earned 19 minutes of league action so far in 2017-18.
The Manchester United academy graduate only recently signed a new four-year contract but it's understood he is growing frustrated at a lack of opportunities.
Reports claim that Jokanovic is ready to swoop for his former recruit as he looks to bolster his Fulham squad in the hope of winning promotion to the Premier League.