'There are very few in his position who can do that'
Number eight Dombrandt completed his hat-trick deep into stoppage time by touching down with an outstretched arm.
Quins had already secured a place in the last 16 but their victory against strong French opposition means they end their Pool B campaign with a perfect record of four wins.
Dombrandt is expected to add to his four England caps during the Six Nations and senior coach Matson said: “I said it right off the bat – he’s a special player who creates moments.
“There are very few in his position who can do that. I think he’d make a difference at the next level.”
The lead changed throughout a fascinating encounter at Twickenham Stoop and Castres appeared to be finishing strongly until Quins rallied with a trademark late fightback.
“The opening 10 minutes for us was misery, they were so good,” Matson said.
“They capitalised on some easy errors from us. But then through the middle of the game how physical were they?
“When they got down to our tryline they scored tries quite easily, which was disappointing and that’s something we’ll reflect on.
“They’re third in the French league, which shows how well they’re going, and for bits they made a misery of the game for us.
“I have to credit this group – and this is one of the reasons we were so successful last year – because when there is two minutes on the clock and we’re in the right end of the field and have the ball there is a genuine belief we can score another try.
“It doesn’t mean my heart wasn’t racing, but it does mean the team has the ability to score one more try. That’s really exciting for this team moving forward.
“I’m not getting used to the tight finishes but I understand that it seems to be the way here. I don’t agree with it but it seems to be the way!”
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