The Nigerian striker continued with his amazing performances, but he was let down by his teammates.
Nigerian forward Emmanuel Dennis scored a first-half goal to give Watford an early lead, but poor Troost-Ekong let his team down after causing a penalty which Bryan Mbeumo converted to help Onyeka's Brentford to a 2-1 victory at the Brentford Community Stadium.
Nigerian duo of Emmanuel Dennis and Troost-Ekong were in action for Watford, while Onyeka was introduced in the second half.
The game started with both sides sharing the ball equally, trying to cancel each other. Bryan Mbeumo was the live wire in attack for the Bees, while the Hornets looked to put Dennis through whenever they had the opportunity.
It didn't take long for the Hornets to get rewarded, as a cross from former Manchester United midfielder Tom Cleverley found Dennis, who leapt high to score his seventh goal of the season.
After they scored, the home side went in search of a goal, but they were refused by the impressive defensive play from Watford, who took the 1-0 lead into the break.
However, the second half started with the home side showing more intent, as they pressed Watford back in their attempts to restore parity.
Troost-Ekong put up an astute show at first, but it didn't last long as Brentford broke Watford's lines in the 84th minute- seven minutes after Onyeka replaced Rico Henry.
Pontus Jansson met a perfectly weighted cross from Marcus Forss to finish calmly behind Daniel Bachmann in goal for the Hornets.
However, the Bees didn't rest on their oars, as they still attacked Watford with intensity.
In Watford's manager Claudio Ranieri's desire to see out the 1-1 score, he hauled off Dennis in stoppage time to replace him with Ozan Tufan.
It didn't look like a bad decision, as the Nigerian forward had an average passing accuracy, and he lost the ball 16 times. Also, two of his attempted dribbles were unsuccessful.
Just as it looked like the Hornets had managed to end their 3-game losing streak with the 1-1 draw, Nigeria international Troost-Ekong committed a defensive blunder as he committed a foul in the box to gift the Bees a penalty in stoppage time.
France international Mbeumo stepped up to penalty duties in the absence of Ivan Toney to send the ball into the bottom right corner. That ensured that Brentford grabbed their third home win of the Premier League season.
Nevertheless, Dennis would take solace in the knowledge that he is only second (12) to Liverpool's Salah(24) in terms of goals involvement this season.
