Sir Alex Ferguson believes sacked Sunderland manager Steve Bruce was not helped by the fact he hails from the Newcastle region, where the locals are known as 'Geordies'.
Bruce, a former player under Ferguson at Old Trafford, left Sunderland on Wednesday after a string of poor results left the Premier League club in 16th place in the table.
The Newcastle v Sunderland football rivalry -- known also as the Tyne v Wear derby due to the cities' respective rivers -- has been rife for decades.
However the rivalry between the north east cities, which sit 12 miles apart, is said to have its roots in a historical conflict surrounding the awarding of coal trade rights to Newcastle by King Charles I nearly 400 years ago.
Ferguson believes Bruce fell foul of the ruthless Tyne v Wear competition.
Ferguson said: "Fans just don't have the patience any longer, just see what happened to Steve Bruce up in the north east where being a Geordie didn't help him.
"In that part of the world they are so passionate and committed to their football team.
"I think Brucie has suffered because of that but that's the kind of supporters we have these days - no patience.
"He rebuilt a whole team and that requires a bit of patience from everyone, including Steve. I think Steve was realising that himself that he was going to have to wait."
He added: "I think losing to Newcastle earlier on in the season didn't help him either because of the way they are in that part of the world but he will be back, Steve. In general his managerial record is very good."
Martin O'Neill is expected to be named as Bruce's successor at Sunderland and Ferguson has no doubts the Ulsterman will make a positive impact.
He added: "Martin has a very good track record. The job he did at Villa Park was excellent, he did a great job at Celtic, he won the League Cup with Leicester so Martin has got the experience and track record to do well.
"If that's the job he's going to I'm sure he will do well."