The much-sought after England Under 21 international had a successful loan spell at the Hornets earlier this season.
"I've spoken to Gareth Southgate at length several times. Adam needs games but Gareth hasn't got the players at the moment," Boothroyd has said.
"We're in there with other clubs but if I could bring him back then I would. It would be good to have him back again."
Crystal Palace, Wolves, Burnley and Sheffield Wednesday are all also reportedly keen on loaning Johnson.