Minutes after the full-time whistle - to chants of 'Leroy out' - the former Torquay manager went into the club's boardroom and quit his post with immediate effect.
The decision appeased the home crowd who voiced their disapproval of the impact Rosenior had had on the club following his appointment in the summer.
But ironically, Brentford started the game brightly. Chris Moore started the game with a genuine sense of urgency to end Brentford's woeful sequence of losses - playing with a mixture of directness and guile.
The former Dagenham striker - bought with the money won by a supporter in a competition run by the league's sponsors - proved to be a real handful to the Crewe defence inside the opening few minutes.
His measured pass sent Brentford captain Kevin O'Connor through and he followed that up with a bursting run through the middle.
But a lapse in concentration at the back proved costly indeed for the luckless west London club when Brentford's defence failed to clear an attack and Crewe striker Luke Varney poked the ball into the bottom corner of the net after five minutes.
The set-back triggered a response from the home side as Brentford pushed numbers into attack in the search for an equaliser.
But the Bees' general naivety at the back, especially when defending set pieces, left them threatened by Crewe's ability to hit them on the break.
Despite not seizing control of the game and dominating possession, Crewe carved out the best opportunities in an energetic first half.
Crewe's three-pronged attack of Varney, Luke Rodgers and Rodney Jack tore into Brentford's nervy defence and the visitors could have racked up four goals before the half-time if they been more clinical.
To sum up Brentford's luck, the home side's best effort in the first half was blocked by one of their own players.
Thomas Pinault's full-blooded volley seemed to be goal-bound until it cannoned off the head of team-mate Clyde Wijnhard and sailed harmlessly over the bar.
In the second half, Crewe should have extended their lead, but Brentford keeper Clark Masters produced a fantastic save from point blank-range to deny Rodgers.
But then the floodgates opened for the away side Ben Rix hammered a fierce shot into the top corner before Varney struck his second and then David Vaughan volleyed Crewe's fourth.