The former Hoops hitman, speaking exclusively to Football Insider, insists the defender isn’t “mature” enough to play for his country.
Steve Clarke axed Ralston from his Scotland squad the World Cup qualifiers against Moldova, which they won 2-0 last night, and Denmark.
The 22-year-old failed to make the cut despite his 22 appearances and four goals in all competitions for the Hoops this season.
The defender’s form has lead to him being handed a new contract by Celtic on 2 November which will keep him at Parkhead until 2025.
McAvennie hasn’t ruled out an international call up in future but he inistsed Ralston has to become a better defender before Clarke will start to consider him.
“I’m sure he will be gutted by that,” McAvennie told Football Insider correspondent Ben Wild.
“For him, it will be a bit of a blow. But, for me, he’s not ready for that.
“I don’t think he’s quite mature enough to hold down that position. I don’t think he can complain really.
“He’s such a young boy and he’s not there yet. You need to be able to defend properly if you are going to play for Scotland.
“I have said before, I don’t think defending is his best attribute. Scotland have to do an awful lot of defending and I don’t think he’s up to that right now.
“He could be in the future, I can’t rule that out. I just don’t think he’s ready.
“I know I will get stick for saying that but he’s just not ready.
“Maybe in a few more years.”