The Belgian international has received criticism for some of his performances this season, most notably from Gary Neville following last month's Merseyside derby.
Former Aston Villa and Manchester United stopper Bosnich believes that Liverpool should sign a quality goalkeeper to push Mignolet.
"He hasn't had enough competition behind him," the Australian told talkSPORT.
"At the time, as a player, you don't enjoy having somebody breathing down your neck for your place. When you look back though, after your career has finished, you realise it contributes to you having the best form of your life.
"It's important for Liverpool. I don't think he is doing ridiculously bad but, for a top goalkeeper, he has to do better. That would push him to a better level."