Kjerrumgaard opened his account after being set up by fellow recent signing Nestory Irankunda, who ran on to Edo Kayembe’s pass down the left and laid the ball across for the Danish striker to slot past keeper Joe Walsh.
And when Louza then delivered a precise cross from the right flank, Kjerrumgaard doubled Watford’s lead by heading in from close range after drifting away from QPR defender Steve Cook.
Rangers were brought back into the game just before the interval when Morgan brilliantly blasted home from near the edge of the penalty area.
They pushed for an equaliser in the second half and Kone, signed from Wycombe this week, was sent on for the final 20 minutes.
The striker almost made the perfect start to his QPR career but his near-post flick from Ilias Chair’s corner was blocked by Bola.
Sam Field also had an opportunity to level late on but shot wide after being set up by teenage left-back Esquerdinha.
Stephan’s side face a tough task in their next match, away to Coventry next Saturday, when Watford will be away at Swansea.