Martin O'Neill admits he wondered if Kieran Tierney would ever be the player he was before being blighted by injury problems, but Celtic's interim manager says the Scotland left-back has "come roaring back" and has been "colossal".
Tierney has scored in two games in a row, Sunday's 2-2 draw with Rangers and Wednesday's 2-1 defeat of Aberdeen at Pittodrie, but is a doubt for this weekend's Scottish Cup quarter-final return to Ibrox.
"The injury he had to his foot is actually clearing up, but he's just generally sore in every aspect," O'Neill said.
"So we'll see how he is tomorrow, but generally speaking, he's done fabulous for us now he's getting almost back to complete fitness, which is great."
Tierney spent six years with Arsenal after leaving Celtic, but the 28-year-old returned in the summer after finding his game-time limited following several injuries and a spell on loan to Real Socieded.
"I think he's been colossal for us," O'Neill, who is in his second spell in interim charge this season, said.
"There were times when I came back in my first spell when I thought would he ever get to the level he was before? And you often wondered if it was fitness alone or the fact he hadn't played for some considerable time."
O'Neill pointed out that Tierney "just couldn't get his career going at all" in the Premier League because of the injuries.
"But he's come roaring back here," he added. "He's been fantastic for us. Selfishly, I'm delighted with it."
