"Our relationship came through young Frank, who I worked with at Chelsea."
Rodgers, who is from Northern Ireland, was youth academy director at Reading before his move to Stamford Bridge in 2004, where he impressed as Chelsea's youth academy director before taking over the reserve side.
Boothroyd left Vicarage Road by mutual consent on 3 November after a poor start to the league campaign, following three-and-a-half years in charge.
Mackay has been in caretaker charge for the last four games and has overseen two wins, including the 3-0 home victory over QPR on Saturday.
"There is absolutely no doubt that Malky's going to be a top manager for the future," Rodgers added.
"He's very important to my management team, and I hope to be able to help him."
The Hornets lie 21st in the Championship, two points clear of the relegation zone, but have reached the Carling Cup quarter-finals, where they will face Tottenham.
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