Glasgow Rangers boss Mark Warburton is close to bringing his 10th player to Ibrox this summer.
Rangers are strongly linked with the 28-year-old striker who is expected to complete his move to Ibrox from Championship side Preston North End this week (BBC Sport).
The English forward has been a prominent player for Preston over the past three years and previously played for Nottingham Forest and Watford, but Johnstone admits that he did not know who he was just six months ago.
Nevertheless, the 62-year-old, who spent a total of 14 years at Ibrox across two spells, believes the Lilywhites hitman can be an "important player" for the Gers after familiarising himself with the player.
He wrote in The Glasgow Evening Times: "Maybe not a household or well-known name, especially in these parts, until the last few weeks. Had I heard of him six months ago? The answer is no.
"But a bit of research, a bit of YouTube watching, and asking a few folk who have seen him, quickly tells you that Rangers could be getting an important player, and the type of player they don’t really have already.
"Garner can score tap-ins, or from 20-yards, with either foot. He also has a good jump on him and can score with his head – not something Rangers have in their armoury right now."
Johnstone is right in his assessment that he will offer Rangers something different up front. As it stands, Mark Warburton has three strikers on the books - Martyn Waghorn, Kenny Miller and Joe Dodoo - and all three are quite similar.
In Garner, they could be signing a centre-forward who offers a bit more by way of power and hold-up play, and such diversity can only help the Scottish giants.
Rangers manager Mark Warburton