The 22-year-old, who plays with Ramsey for Wales, left boyhood club Nottingham Forest for Spurs in a deadline day deal worth £47.5million.
Speaking in his Wales press conference about Johnson's move to Spurs, Ramsey joked, as you would expect from a former Arsenal player: "I wish it weren't to Tottenham.
"For him it's a brilliant move. The potential he has is quite scary and hopefully he can take those next steps and go to the next level.
"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."
READ MORE TOTTENHAM
Johnson faces awkward Richarlison reunion after reacting angrily to striker's showboating
Tottenham end transfer window on a high as Brennan Johnson arrives from Nottingham Forest
Johnson had scored 29 goals in 109 appearances for Forest, playing a key role in helping Steve Cooper's side to secure promotion from the Championship, before helping them to consolidate their Premier League status last term.
Having also won 19 caps for Wales, Johnson got his first taste of life at the highest level with three appearances at the World Cup in Qatar.
Tottenham manager Ange Postecoglou also explained the reasons behind the move, saying: "He's just another exciting young players.
"He's a real threat in the front third, he's different from what we've already got, he's young and ambitious. It's no secret that's the model for me.
Most read in Football
Police investigate Keane assault, Sancho hits back, Messi lives up to Hollywood billing
Blood seen on the floor after altercation between fan and Roy Keane
How to watch Ukraine vs England: TV channel, live stream and talkSPORT coverage
Lionel Messi booed by LA fans in front of Hollywood A-listers and Prince Harry
"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."