Speaking to Dutch outlet De Telegraaf ahead of the World Cup, the Netherlands international said the departure was entirely consistent with what he witnessed during his first and only season at the Etihad Stadium.
"He's a very intense coach and a football brain," Reijnders said. "In a way, it's a shame I only got to work with him for one season. But it's his choice."
He added the detail that explained the decision most clearly.
"He no longer has the energy to disappoint players who aren't selected or don't play. After ten years, that's enough. I completely understand him. When I talk about an 'intense coach', it's in a very positive sense — he seeks to get the most out of his squad."
Guardiola himself had been candid about the same feeling. Asked at his final press conference as Manchester City manager whether fading energy was behind the decision, he said: "Absolutely. I feel I will not have the energy that is required every three days — the expectations to fight for titles, to be in front of the players. I know myself."
The 55-year-old stepped down after a 2-1 defeat to Aston Villa on the final day, ending a decade that produced 17 major trophies, including six Premier League titles, the Champions League and four consecutive league titles between 2018 and 2022.
Reijnders joined City from AC Milan in the summer of 2025 for £46 million and was used primarily as a squad player by Guardiola in the second half of the season. His role under incoming manager Enzo Maresca is expected to be significantly more prominent — though reports this week have linked him with a summer exit to Atlético Madrid if he remains on the periphery of City's plans.
Guardiola, who took charge for the last time on May 24, has said he intends to rest for an extended period before considering his next move. Friends of the manager have told the Daily Mail he has privately expressed interest in the England job at some future point. For now, as Reijnders' comments confirmed, the hunger that built one of football's great dynasties ran its natural course.