De Zerbi said: "We are losing a great man and a great player. It was his decision and we must respect it. He was a fundamental player for me during my stay here. All starters Adam Lallana, Adam Webster, Pascal Gross, they have followed me from day one, and for us it will be a big loss.
"He is very intelligent, he loves football, he knows the game both as a player and as a coach, but I think he could definitely play another season in the Premier League."
Chairman Tony Bloom also paid tribute: "Adam was a crucial signing for us, bringing us high-level experience at a critical time. His ability was clear to all, but what wasn't was evident to those watching it was the way in which Adam helped raise standards and expectations in the dressing room and throughout the club.
"The fans, and the media in particular, got to see him and, more importantly, hear him during those lockdown games, when you could feel how vocal Adam was on the pitch in terms of leadership qualities. He has been a great servant to this club and a major factor in our recent progress, not least in qualifying for European competition for the first time in the club's history.
"On behalf of all Albion fans I would like to thank Adam for his time here and wish him well in the next phase of his career."