He had been stung by criticism of his players for failing to find the net in five of their previous seven games.
The Albion boss said: "I've been very upset. People expect us to be 10 points ahead of everybody else.
"But we are a young team. In our last few games we've struggled to score, but we created a lot of chances.
"So I'm not particularly happy - I'd rather win five games 1-0 than one game 5-0, because it's only three points." Di Matteo's mood was lightened by a man of the match show by almost forgotten striker Simon Cox, making only his second league start since his £2million move from Swindon.
Di Matteo said: "Over the last couple of weeks Simon has looked sharp in training and he scored for the reserves on Monday.
"It takes time to adjust to a higher division but I was pleased with him today and hopefully that's the first of many performances like that." Cox's 68th-minute goal completed the drubbing of a Watford side unbeaten in their previous six away games.
Albion took the lead after five minutes when Jonas Olsson headed in Graeme Dorrans' free-kick.
Dorrans added the second from the penalty spot on 18 minutes after Adrian Mariappa fouled Luke Moore.
Moore himself added the third four minutes after the break and Gianni Zuiverloon hit the fourth goal on 64 minutes before Cox struck.
Watford boss Malky Mackay admitted: "It was very disappointing."
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