Reina revealed that the Spanish manager had lost the dressing room and the time was right for him to leave.
"When Rafa left Liverpool at the end of the 2009-10 season, I knew that it was the right time for him to go," Reina said.
"It hurts me to say that, but the wheels had come off by then and there was no guarantee that he was going to be able to put them back on.
"I know that I will be for ever in his debt for what he has done for my football career, and it is never nice when a manager leaves or loses his job, but there are times when, for whatever reason, the methods that have been bringing a manager so much success just stop working.
"When this happens, the club has to make a decision about whether you will be able to get back to winning ways again, or if it is time to look for a new manager who might be able to freshen things up, just by having a different approach.
"Liverpool wanted to go down the latter route and it was probably the right decision.
"I was upset, obviously, because it is always sad when a manager loses his job, but even more so when he has been as important to your career as Rafa has been to mine.
"But I also thought half of the dressing room was not happy and so, probably, for the club if not for me personally, it was the best outcome for everyone."
Reina explained that he first began to realise the Reds were in serious trouble when they lost 3-0 to Espanyol in a pre-season friendly at the start of August 2009.
"They beat us 3-0 and could have scored more. When I came off the pitch there was a part of me that thought, 'If we carry on like this, we are going to get relegated'," he said.
"It was as if someone had flicked a switch at the end of the previous season and we had gone from being a really strong team to a really weak one.
"It was clear that we were nowhere near the level that we wanted to be at. When there are problems at a big club and the atmosphere turns, one of the first comments to be made is always that the manager has lost the dressing room.
"In this case there were still players who supported the manager, but obviously there were others who were not too happy with him for different reasons.
"My own opinion was clear - I liked Rafa and continued to support him - but I cannot speak for everyone.
"Sadly for Rafa, we went from second place in the Premier League one season to seventh place the next and that was always going to result in him coming under pressure.
Reina, who is the quickest goalkeeper in Liverpool history to keep a half century of clean sheets, said Liverpool suffered because they failed to properly fill the gap left by the departures of quality players like Xabi Alonso, Peter Crouch and Jermaine Pennant.
"Signings were made with the idea of making improvements to the team, but the reality was that the ones who came in were not of the same standard as the ones who had left," he said.
"You can look at the players who came in and ask why they didn't deliver, because no footballer can ever be free of responsibility.
"But, in football, the buck always stops with the manager. If he makes signings that don't work out then it won't be long before the people who run the club are going to ask questions.
"Had someone said to me at the end of the 2008-09 season, when we came so close to winning the league, that just 12 months later the manager would be gone, I wouldn't have believed them," he added.
"But that is football. It is not about what you might have done in the past, it is about what you are doing in the here and now and what you are going to do in the future.
"None of us can live on past glories.
"The moment any of us think that we can is the moment that decisions about our future are taken out of our hands - if I have a really poor season in goal, the chances are that Liverpool will start looking at the possibility of replacing me."
Benitez left Liverpool at the end of the 2009/10 season and was replaced by Roy Hodgson. The former Fulham manager was sacked midway through to the 2010/11 season allowing current manager Kenny Dalglish to assume the hot seat.