The Celtic boss likened the 20-year-old to a "train".
The goalkeeper joined the Owls from their Championship rivals earlier this week.
The Finland international was unhappy with his performance in the pre-season win over Port Vale on Tuesday.
The Leicester City scout completed a move to Everton on Wednesday.
Newcastle United and Aston Villa are currently among the favourites to win the Championship next season.
Celtic take on Lincoln Red Imps in the second leg of their Champions League qualifier later this evening.
Button completed a move to local rivals Fulham on Tuesday.
The Dutch playmaker spent two successful years at Spurs.
The Magpies have already signed eight new players this summer.
Mark Warburton has signed nine players for Ibrox side Glasgow Rangers this summer.
Labour MPs supporting Jeremy Corbyn are urgently seeking ways to avoid a historic split after their leader’s refusal to resign set the party on track for a ferocious leadership battle.
Senior figures from Labour and the Scottish National party are considering calls for legal action against Tony Blair if the former prime minister faces severe criticisms from the long-awaited inquiry into the war in Iraq.