The Rangers number one signed a 12-month extension on Thursday, keeping him at Ibrox until summer 2022.
But McAllister, who hung up his own boots aged 39, believes McGregor's “meticulous” application to the game can see him outlast the assistant manager and play beyond the latest deal into his forties.
The Rangers assistant said: “I have been a player that has played pretty long and you get to a point where maybe his body will tell him. At the moment, there are no signs of that. He is in great condition and it will be entirely up to Allan. It comes from his gut and his desire to keep going.
“[The new contract was] a no-brainer, absolutely, and he thoroughly deserves it as well.
"He’s a massive work-horse. The way he comes in every day, his preparation for games is pretty stunning. All goalkeepers work extremely hard but Allan is something different and he is meticulous in his preparation.
"The saves that he makes, whether first minute or last minute, against teams at the bottom of the league or top or in a European tie, his concentration and focus is there to be seen.”
McGregor followed Steven Davis in renewing his deal and McAllister added: “I’m really pleased he will be with us again. You do tend to look down the spine of a team so when you go McGregor and have Davis in that line as well, that bodes well.”