Watford suffered their early blow when Bayo pulled up injured off the ball, leading to an early introduction for Rajović.
Tom Dele-Bashiru’s loose pass led to a Foxes breakaway when the game resumed but Abdul Fatawu’s attempted ball to Daka was well cut out by Hoedt.
After nine minutes, the Foxes took the lead from the spot when Giorgi Chakvetadze's outstretched leg was adjudged to have brought down Dennis Praet in the box.
Daka stepped up and rifled low past Hamer and into the bottom right corner to give his side the perfect start.
Hamer was quickly needed to rush out and avert the danger after the Hornets played him into trouble, before Daka blazed over the bar from the edge of the area.
On 19 minutes, a promising Hornets counter-attack saw Jamal Lewis swing in a cross but Matheus Martins was eased out at the back post.
At the other end, Kiernan Dewsbury-Hall charged powerfully into the box before lashing his shot wide of the near post under close attention from Watford defenders.
The Golden Boys more than once made life difficult for themselves by giving the ball away and another Leicester break resulted in James Justin sending a cross too high when nicely placed.
Ismaël Koné then orchestrated a bright moment for the hosts, switching the ball onto his right foot and drilling a low shot which deflected just wide before the referee denied a corner after blowing for a foul.