International friendly match: Wales v South Korea
Venue: Cardiff City Stadium Date: Thursday, 7 September Kick-off: 19:45 BST
Coverage: Live on BBC Radio Wales, BBC Radio Cymru, BBC Radio 5 Live Sports Extra, BBC Sounds, BBC Sport website and app, plus live text
Highlights: Match of the Day Wales. BBC One Wales from 22:40 BST and later on demand
Wales forward Johnson, 22, made a deadline day move from Nottingham Forest in a deal worth in excess of £45m.
His signing has been seen as part of Tottenham's efforts to replace the attacking void left in the wake of record goalscorer Kane's move to Bayern Munich.
Son says he is looking forward to lining up with Johnson and seeing him up close when South Korea face Wales in a friendly at Cardiff City Stadium on Thursday.
"I think that by coming to Spurs (Johnson) wants to improve and go to the next step," said South Korea captain Son.
"He will feel the pressure as well but I can't wait to have him in our team, playing even better football, better attacking, better fast football, but I think everyone has to take that responsibility.
"You don't want to let Harry go because he's one of best players in world.
"When you have Harry Kane on your team you have 20-30 goals in your pocket. You have these goals guaranteed, but I think the players have said 'Okay, everyone has to take a big step forward to move this club forward'.
"Everyone has to take a big responsibility because certainly we've lost 30 goals. Now everybody has to step up to make those 30 goals together which is very important."
Ange Postecoglou's side have won all three games since Kane's departure, including a 5-2 win over Burnley before the international break. The game at Turf Moor saw newly installed captain Son grab his first goals of the season with a hat-trick.
Johnson missed that match having completed his move too late to be involved, but will add pace and his own eye for goal to Tottenham's attack.
Wales captain Aaron Ramsey insisted earlier this week Johnson was ready for the step up, and now Son also backed the 20-cap international's ability.
"Brennan has shown his quality last year with Forest so I can't wait to see him with my own eyes," Son, 31, said. "He is a very good player, I think he has a very good talent.
"I haven't played with him and I haven't seen him in action but I can't wait to see him on Thursday.
"Obviously we are playing against each other but then a few days later we are playing together and I can't wait to play with him."
And Son's international manager, former Spurs striker Jurgen Klinsmann, says he does not rule out Tottenham challenging for silverware this season - and says they are Arsenal's equal.
"We all know what's happening in the Premier League and how dominant Manchester City are at the moment and how difficult it is," said the German World Cup winner. "But look at last year, what Arsenal did was amazing - but I see a team that does not have less quality than Arsenal in this Spurs side.
"They have a fantastic coach who is a good people person - so why not surprise people? His goal like his entire team is to win trophies - yes it's been a long time but you start every year with this goal in mind and as a follower and a fan I will keep my fingers crossed."
Klinsmann will also hope for better luck in his current role, failing to win any of his four matches since being appointed in February with the aim of ending a 60-year wait for the Asian Cup.
The former US and Germany national manager said the friendly in Cardiff is part of the preparation for January's tournament in Qatar.
On facing Wales in Cardiff, he added: "We've done our homework, we're not underestimating the Welsh team for a second, but we will try to play to our strengths. We have a couple of talented players with us and it's a good opportunity for us. I'm convinced we'll show you some good quality of football."
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