Beppe Sannino said he felt sorry for Samba Diakite after the midfielder was sent off on his first start for the Hornets but the Watford boss believes the red card brought the best out of his team.
Diakite was shown a straight red in the second period of the 1-0 win over Middlesbrough after a reckless two-footed challenge.
His dismissal threatened to ruin Watford's hopes of sealing a valuable win but Sannino was sympathetic towards his player.
"I feel sorry for Diakite because he has been sent off and it was the first game he started in a Watford shirt," Sannino said.
"I am not sorry for the tackle but for him because it was an important moment of the game but we look forward."
Boro piled on the pressure in the second period and Fernando Forestieri was forced to drop into midfield to make up for Diakite's error.
Sannino believes the red card revealed his team's character and singled out Forestieri for special praise.
"This sending off allowed my team to show good spirit and Forestieri was able to drop into midfield and do another role so I'm happy about that," said the Italian.
"He was still winning a lot of tackles in the defensive phase so I think Forestieri understood today the soul of this team.
"Then behind Forestieri too all my team were able to show me that soul."