The striker, 22, joined Spurs on deadline day for £45million and could come up against Ramsey’s old team, Arsenal, in the north London derby later this month.
“I wish it weren't to Tottenham,” Ramsey joked to reporters when asked about the deal.
Ahead of Wales’ games against South Korea and Latvia, the Dragons skipper described Johnson’s switch as a “brilliant” chance to reach his potential.
“The potential he has is quite scary and hopefully he can take those next steps and go to the next level,” he continued.
“He's going to be important for Wales as well. He's going to be huge for us over the years to come and we're all excited that he has another platform to go on and improve himself again.”
Ange Postecoglou was unable to give Johnson his debut against Burnley on the weekend but explained why Tottenham have invested heavily in the “exciting” attacker.
“He's a real threat in the front third, he's different from what we've already got, he's young and ambitious,” said the Australian. “It's no secret that's the model for me.
“It's just as much about the person as well as the footballer, and looking at him he's going to fit in really well with this group. He's ambitious and hungry and wants to take his game to the next level.
“The technical and the physical attributes he has, he should fit in really well.”