Chelsea currently sit fourth in the table, six points ahead of United, and Bosnich feels that a nine point gap would be too large to bridge later in the season.
The former Manchester United goalkeeper told Sky Sports News: "I don't think it's win or bust but you could lean towards it being lose or bust.
"I think it's a must not lose. The gap will be a little bit far to catch up if they lose, but not win or bust no. They're not far off and Chelsea will be looking to this with Tottenham breathing down their necks and they're at home.
"They both had a week's break so it will be a very important game for both sides."
The Australia international made 38 appearances for United across two spells and seven for Chelsea, in addition to playing 207 matches for Aston Villa. body check tags ::