Bosnich claims Ferguson must decide which of his goalkeepers - either David de Gea or Anders Lindegaard - is the club's first choice keeper to avoid a repeat of the defensive lapses the team is currently experiencing.
De Gea arrived from Atletico Madrid in the summer for around £19million but has struggled to establish himself as United's undisputed number one and was culpable in their 3-2 defeat to Blackburn at Old Trafford on New Year's Eve.
On the other hand, Lindegaard has looked fairly solid in his limited starts for the Red Devils but was involved in United's 3-0 loss at Newcastle on Wednesday which featured a bad mistake from defender Phil Jones who headed Tim Krul's deep free-kick into his own net in the final minute of normal time.
"Manchester City are looking like they could easily run away with the league," Bosnich told talkSPORT.
"He [Sir Alex Ferguson] needs to make a decision and stick with one, you couldn't really fault Lindegaard against Newcastle but that misunderstanding with Phil Jones was because they haven't played together, week in and week out.
"I understand you have to rotate players but in the key positions, and being a goalkeeper is a really key position, that understanding must be second nature and that was the result of the third goal. It does take a little bit of time to get used to and in the Premier League you don't have that sort of time.
"He's got to decide who he's going to stick with, whether it's De Gea or Lindegaard, regardless of how much money he's spent on De Gea if Man United are going to win any trophies."
United are aiming to avoid losing three successive games for the first time in almost 11 years when they play at Manchester City on Sunday in the FA Cup.
United have not suffered three defeats in a row since the last three games of the 2000/01 campaign, when the Premier League title had already been won.