Tottenham Hotspur centre-back Jan Vertonghen is delighted to see Son Heung-Min living up to expectations at the club, and says the forward is a defender's nightmare.

Son, 24, equalled last season's Premier League tally of four goals on Saturday with a second double in a fortnight as Tottenham won 2-1 at Middlesbrough to maintain their unbeaten start to the domestic season.

The South Korea international admitted after the game he had asked to leave Spurs in the summer after a difficult maiden season in which he started just 13 league games, but Vertonghen, 29, is pleased he stuck around.

"Nothing much has changed [from last season] but then he scored that double at Stoke [earlier this month] and that gave him so much confidence," Vertonghen told reporters.

"He's making his one-on-ones again, he's scoring goals, making assists, giving the right-backs a hard time and that's how we want to see him. He can score goals with his right and left, he's very direct and he works very hard. It's a pain to mark a guy like that."

Vertonghen believes Son can help replace Harry Kane's goals while the striker is sidelined with ankle ligament damage.

"Before Harry was our main goal scorer but now we expect goals from other players and it's great to see guys like Christian [Eriksen], Vincent [Janssen] and Sonny can step up," he said. "We need that and it's great to see them doing that already.

"Obviously he [Son] came with big expectations last season, and he did well, but he expected more. This season he has really stepped up and he's been so important for us this season and it's great to see him at his best again.

Son Heung-Min has scored four Premier League goals in a fortnight. IAN KINGTON/AFP/Getty Images

"He's a great guy. He's always smiling, everyone loves him. He's a great character around the training ground, he's good with everyone, very social. Everyone just loves him."

Vertonghen's centre-back partner Toby Alderweireld added of Son: "It's easy to say when he's got three man of the match [awards from the last three games], but on the training ground he always shows how much quality he has. In small-sided games, he's always one of the best. He shows how good he is.

"Finally he's got the confidence and he's starting, and when you score the confidence comes from yourself. I'm very happy for him."

Between scoring a fine double in the 4-0 win at Stoke and an impressive display in the 1-0 victory over Sunderland, Son was hauled off at half-time of Spurs' 2-1 defeat to Monaco in the Champions League.

However, he is expected to keep his place for Tuesday's second group-stage game at CSKA Moscow, and Vertonghen says Spurs are desperate to secure all three points in Russia.

"In Champions League home games, you expect to win," he said. "If you don't, you have to get your points somewhere else and that's what we have to do now.

"We go there for the win. We have the team to win. We expected much more from the last game and we need to go up a level in the Champions League as well. Maybe there's a difference to some of the Premier League games.

"We had a good game [against Monaco] and it sounds arrogant but I think we were best in possession and created some good opportunities. Before they scored, Sonny [Son] had a great chance and we need to finish them. We need to be clinical."