27-year-old goalkeeper Johnstone joined Premier League giants Manchester United as a ten-year-old and remained on their books until 2018.
Although the Englishman could not make a senior appearance for the Red Devils, he spent time training with the first team squad, including De Gea.
Recalling his experience training with the Spaniard, Johnstone explained that spending time with him was a very good training curve.
The West Brom star hailed how De Gea’s carries himself on and off the pitch and also revealed that they remain in touch even after his Old Trafford exit.
“Just watching him every day, everything, in the gym, just around the place, training, before games“, Johnstone told WBA TV.
“When we travelled, very relaxed. Very confident in his own ability and obviously very good.
“Warm-ups, watching him… it can’t get much better than that, learning from him every single day.
“Great person. It is good. We still keep in contact now.
“He did message me last year and he was like ‘I want to see you in the Premier League’.
“It is nice to get messages like that, do you know what I mean?
“[It was a] very good learning curve.“
Johnstone will reunite with De Gea when West Brom visit Manchester United in the league on Saturday.