Spencer, a former Blues and All Blacks first-five, said he wouldn't like to be a selector with the likes of Beauden Barrett, Aaron Cruden and Colin Slade putting in sustained challenges to Carter.
"I think Dan at the moment, to be honest, is going to struggle," Spencer said. "He just hasn't been right for the last two or three years. He's injury prone and there are no guarantees, especially now for him.
''He's under pressure. He's got three guys underneath him who can do the job just as good. He has to come back and have a big Super Rugby campaign. If he can do that I think he'll grow in confidence and be right for the World Cup. But when you can choose from four fly-halves, it's not too bad when everyone else seems to be struggling."
Spencer, coaching Currie Cup team the Eastern Province Kings in South Africa, said he was impressed with the All Blacks' consistency since their World Cup win in 2011.
"It's interesting when you're out of the country and you're probably one of the few All Black supporters around and people talk about how the All Blacks lost - that might be the case but they seem to be able to pull it off when it matters."
