Manchester United manager Alex Ferguson has questioned Kevin Keegan's decision to return to Newcastle United for a second spell in charge.
"Sometimes it is wrong to go back to a club for a second spell," Ferguson was quoted as saying in Thursday's Daily Telegraph newspaper.
"There are examples of managers doing it and having relative success but no outstanding success, so I will be interested to see how Kevin copes with it."
Former Newcastle player Keegan, who previously managed the club for five years in the 1990's, took over from Sam Allardyce earlier this month.
He almost took Newcastle to the Premier League title in 1996 but the team blew a 12-point lead and Keegan famously lost his temper in a television rant at Ferguson before finishing runners-up to United.
Keegan has not coached since leaving Manchester City in 2005 and Ferguson was unsure what effect that would have on Newcastle.
"Some managers have taken sabbaticals and that's been a problem for them," Ferguson said.
"It's difficult to say how it will affect him not having seen any games in three years. He's had a rest and he will have the freshness that brings, so who knows?
"But the game has definitely changed and it's changing all the time," Ferguson added.
Newcastle, 12th in the Premier League, 30 points behind leaders United, have not won or scored a goal in the former England manager's three matches in charge so far.