Celtic brought in the 24-year-old Canadian right-back from Montreal Impact in January in the hope of replacing Josip Juranovic, who joined Union Berlin in the same window.
Johnston made his Celtic debut against Rangers in early January and has been a regular in Ange Postecoglou’s starting line-up since then.
Ferguson stated that Johnston is a brilliant athlete and admitted that the Celtic right-back’s relentless work ethic on the right side has impressed him.
The former Scotland star is of the view that Celtic have done a brilliant job in replacing Juranovic, who according to Ferguson was a top full-back, with Johnston.
Ferguson added that the Canadian has handled himself very well so far and believes that Johnston is among the latest of Postecoglou’s great signings.
“Yes, he is a good athlete, and that’s one thing that strikes me about keeping up and down over the right side, no problem”, Ferguson said on Go Radio.
“Juranovic for me was a top full-back, and that’s just the way Celtic operate.
“They bought Juranovic for a couple of million quid, sold him for seven and a half million plus add-ons, and then they brought in Alistair Johnston for two and a half or three million pounds.
“Real good business, and to be fair to him, I know he has been there for a number of months and his first game was baptism of fire at Ibrox, and he handled that very well.
“He has been another really good signing for Postecoglou.”
Johnstone has featured in eleven games for Celtic so far this season and scored his first goal for the club at the weekend against St. Mirren.