It marked the defender's full Premier League debut for Spurs.
Postecoglou said: "I don't think it's a game to talk about individuals, because whether it's Radu's first game or Sonny's 500th, it doesn't really matter.
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"What matters is that as a group we just didn't reach the levels we need to in order to compete so for us, there's obviously a lesson in that. It's part of our growth.
"You've got to feel, all these things keep saying, they're not firsts but they're things you've got to go through if you want to get growth and it's how you react to them and that's our challenge now."