It was a bold statement from a goalkeeper who had never before played in the Premier League, writes Joseph Masi.
But Johnstone has achieved his aim in spectacular fashion with the 28-year-old having made it into the Three Lions squad that will contest the European Championships.
It is true that an injury to Burnley’s Nick Pope has made the path into that squad easier to tread.
But a look at Johnstone’s numbers this season prove he thoroughly deserves his place.
For starters, the shot-stopper has made more saves than any other goalkeeper in the Premier League this season.
In total, Johnstone has made 166 saves.
That’s comfortably more than England rivals Aaron Ramsdale (147), Nick Pope (114) and Jordan Pickford (90).
But being a goalkeeper isn’t just about keeping the ball out, it’s the one position where you cannot afford to make any mistakes.
And according to the Premier League’s official statistics, Johnstone didn’t make any errors leading to goals last season.
There are fans who, understandably, will feel he should have done better with Kalvin Phillips free-kick for Leeds on the final day of the season.
But even allowing for that goal, Johnstone made fewer mistakes than Ramsdale, Pope and Pickford – who all made two errors leading to a goal.
That stat particularly concerning when it comes to Henderson as the Manchester United man only started 13 top flight matches.
Another area Johnstone toppled his England rivals last season was with penalty saves.
Only Johnstone and West Ham’s Lukasz Fabianski managed to save two spot-kicks last year.
Henderson didn’t face any. But Pickford, Pope and Ramsdale weren’t able to keep out any of the penalties they faced.
One thing – somewhat surprisingly – Johnstone does need to improve though is his distribution.
Southgate famously made Pickford his number one because of how good he is with the ball at his feet.
Johnstone is another goalkeeper who is considered to be good at playing out from the back – with Albion fans used to seeing him ping cross-field balls out to Darnell Furlong.
But his pass completion rate for last season stands at just 43.2 per cent.
Pickford’s is a staggering 61.8 per cent.
But Pope (44.6 per cent) and Ramsdale (43.4 per cent) both proved to be better with the ball at their feet last season.
Interestingly, though, Johnstone averaged 30 passes per game while Ramssale averaged 27 and Pope 26.
Pickford, on average, attempted 32 passes per game.