Guardiola deemed Hart not worthy of a starting spot in his City line-up, electing to use Willy Caballero at the start of the season before buying Claudio Bravo from Barcelona.
The decision seemed to be based on Hart's ability - or lack thereof - to play the ball out from the back with his feet in the manner Guardiola wanted.
Within a few months, Hart had gone from first to third-choice goalkeeper at the Etihad Stadium, and was sent out on loan to Serie A outfit Torino.
However, Reina - who worked under Guardiola at Bayern Munich - insists Hart is still a "top-class" goalkeeper.
"The game has changed so much for keepers because a lot of managers now expect us to be able to start play from the back as well as keep goals out," Reina told Joe.co.uk.
"I don't think we should be called goalkeepers any more because we are more like goal players.
"That is more the case if you are in a Guardiola team than if you are in any other and that's why he felt signing Claudio Bravo from Barcelona was so important to the way he wants City to play.
"I have seen people say that Hart isn't good with his feet but that isn't true. In terms of the speed and precision of his passes he is very good.
"The difference is that Bravo has been a goal player for many years and does not have to learn Guardiola's methods and that is why Hart was replaced.
"But he remains a top -class keeper. There is no question about that."
Hart is in line to make his Serie A debut when Torino tackle Atalanta on Saturday, and Reina had some advice for the 29-year-old.
"It is more tactical and less physical here," the Napoli goalkeeper said. "If you go for a high ball under pressure and don't claim it there's a strong possibility that you will be given a free-kick because the keepers get much more protection here than they do in England.
"Every team plays its own way but I would say that after Napoli, last season Torino were one of the teams who play shortest; they liked to play from the back and that means the keeper has to be involved.
"But they have changed managers with Sinisa Mihajlovic replacing Giampiero Ventura so maybe their style will become a little bit different.
"But however they play I wouldn't expect Hart to have too many problems because he has played at the top level for so long."
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