The Red Devils are thought to be in the market for a new goalkeeper due to the uncertainty over David de Gea's future.
"I'm not 100% certain about [Cillessen]. I think he did OK at the World Cup [for Holland], but he's not one who stands out in terms of being dominant," the Australian told talkSPORT.
"The English Premier League is definitely the most physically demanding in the world and it takes foreign players quite a while to adjust to that.
"We saw during De Gea's first season, it was a bit of a struggle. Lloris knows the Premier League inside out and he's had success."
Bosnich spent four years at United across two different spells, while also playing for both Aston Villa and Chelsea.