Ighalo sealed a last-gasp Deadline Day loan move to United from Chinese Super League side Shanghai Shenhua.
United turned their attention to Ighalo after failing with late attempts to sign Salomon Rondon and Josh King.
The 30-year-old arrived in Manchester on Sunday with United hoping he can provide a cutting edge after they failed to score in three successive Premier League matches for the first time since October 2016.
“This was an emergency signing,” Bosnich who won the Intercontinental Cup (defunct) and Premier League title with United in 1999 and 2000 respectively, told Sky Sports News.
“I can understand a lot of people talking about a scatter gun approach, well it had to be with Marcus Rashford being out.
“The top four is still possible for Manchester United, they had to go and get somebody.
“The boy that’s come in (Ighalo) has got nothing to lose, nobody expects anything unbelievable from him but they definitely needed somebody in that position.
“You never know. A long, long time ago Manchester United signed a certain Frenchman (Eric Cantona) who no one really expected much from and he ended up propelling them to one of their most successful seasons of all time.”
United’s goalless draw with Wolves on Saturday leaves them seventh in the table, six points off the top four.
But the 48-year-old former Australian international voted Oceania Goalkeeper of the Century in 1999, believes they can still qualify for Champions League football next season.
“I think they can, there’s no doubt about that,” he added..
“Six points at this stage of the season for me is still nothing.
“The big question of them not scoring goals is the way that they’re playing.
“People would have a lot more confidence if they had been watching performances and saying okay this team is actually going somewhere.
“I do believe that they are. Whether or not they’ll be able creep into that top four at the end of the season is a different question entirely.
“There’s a still a chance, regardless of what anyone says they’re still in the mix.”