Antonio Rudiger will join Real Madrid in the summer.
The German defender has decided to not renew his contract with the Blues and will depart Stamford Bridge by the end of this season.
Chelsea are yet to find a new owner for the club and they are currently unable to offer any new deals to their players.
Merson said in his Daily Star column: “Antonio Rudiger is the one player Chelsea couldn’t afford to lose, but he clearly didn’t love the club enough to stay.
“He is risking the wrath of the fans now because he is turning down a lot of money and leaving them in the lurch, but let’s be honest: it’s been on the cards for a long, long time.
“I called this one months ago. I don’t think Rudiger ever had any intention of signing; not after he was frozen out by Frank Lampard.
“I know plenty has changed since then. Thomas Tuchel came in, brought him back into the team, and they won the Champions League.
“But he is irreplaceable really. Chelsea can’t replace him. No-one has bought the club yet. They are still under sanctions. They can’t even line someone up.
“Rudiger is absolutely perfect for the way they play. He goes and squeezes players. He’s full of energy. A different class. And they say he’s a leader in the dressing room.”