Johnstone put in a solid display against Nottingham Forest in a 1-1 draw in the Premier League.
Johnstone, who had arrived a day ago from Crystal Palace, knew his experience would pay dividends.
The 31-year-old told club media: “I loved it. Obviously, I've only been here two days.
“They just showed me the goalkeeper coach and little bit of a team on Friday, so still getting to know my teammates and how we want to do things. But once you step on the pitch, I think your natural instincts come into it. I've played many games and it was good, not loads to do, just a couple of nice saves.
“It’s been a pretty busy few days, coming into a new club late on and trying to get used to the lads, get used to names and faces and the way we want to play. It’s going to take some time and the lads have made me feel settled in the first two days and straight into the team for a big game.”