Johnston played the full 90 minutes as Celtic beat Dundee 1-0 on Monday, with Everton believed to have had a scout present at Parkhead. Manager Martin O'Neill revealed in his pre-match press conference on Friday that David Moyes had telephoned him personally to flag that Everton were preparing an approach, though the two clubs are yet to agree a fee for the Canada World Cup star.
Reports on the valuation vary widely, with some outlets citing figures as low as £12m and others closer to £25m, though The Athletic puts Celtic's expectation at around £15m; Johnston joined from CF Montreal in January 2023 for around £3.5m and has three years left on his contract, giving Celtic no obligation to sell at a discount.
Celtic's priority for now is Europe. They were drawn against Austrian champions LASK in the Champions League play-off round on the same day as the Dundee match, with a home leg on 19 August followed by a trip to Linz on 25 August, as they look to avoid last year's disappointment of missing out on the lucrative group stage.
That fixture list makes the club's hierarchy reluctant to sell key players unless their valuations are met or suitable replacements are found first.
Asked after the game whether it was more important to keep his squad together or add to it before facing LASK, O'Neill said: "It would be nice to do both to tell you the truth, it would be really nice."
He noted that speculation over Johnston and fellow target Arne Engels had not affected either player's application against Dundee: "Ali, it never bothered him at all and that was great. He said: 'Yeah, I'll play.' It is disconcerting at times when you think about it, and you're worried about injury and all that type of stuff.
"But I'm hoping that it would be nice if all that transfer speculation went away, but I'm sure it probably won't. It would be nice for it to go away and to get a few more players in, which we're trying to do."
Everton have also considered alternatives at right-back, including RB Leipzig's Germany international Ridle Baku, while Jake O'Brien and Nathan Patterson remain their current options in the position, with O'Brien potentially able to shift into his more natural centre-back role should Johnston's move go through.