The ex-Red Devils shot stopper has recommended the club's owners give David Moyes as much money as he needs to save the faltering season.
United is currently seventh in the Premier League - winning just half of its opening 20 games - and was knocked out of the FA Cup by Swansea City on Sunday on home turf.
Bosnich, formerly a Red Devils' keeper, believes manager David Moyes will have to open up the check book if the team is going to claw back the 11 point deficit on leaders Arsenal.
"Man United’s problems can be traced back to their lack of activity in the transfer window last summer," the Australian told TalkSPORT. "You have to improve your squad every year if you’re going to stay on the top. The Glazers, Ed Woodward or whoever is calling the shots, have really got to stand up in this January transfer window and give Moyes funds to get the players he really wants.
"This season is heading towards disaster. They might not finish in the top four. They really need to say: 'This season is salvageable, we might not win the title, but we could finish in the top four but we need to identify big players, players of Manchester United’s caliber and spend some money.'"
Bosnich spent two spells at the Old Trafford outfit, playing a total of 33 times under Sir Alex Ferguson, and also spending time at fellow Premier League clubs Aston Villa and Chelsea.