Pochettino is thought to be a leading contender to take over at Old Trafford this summer when United announce the long-term successor to the sacked Ole Gunnar Solskjaer.
Merson wrote for Sky Sports: "I don't understand why Manchester United would be waiting for Mauricio Pochettino. I don't get it. If Pochettino doesn't win the Champions League with Paris Saint-Germain, why would they be going in for him?
"You've got to remember he failed to win the French league last year, too. That's virtually impossible to do. Everybody keeps saying they need to go out and get Pochettino but I don't understand why.
"It does my head in. It's all I ever hear. What did he do? He got Tottenham to the final of the Champions League and his side finished third one year in the Premier League.
"If I was in charge at United, I'd be going for Rodgers or Graham Potter now. People are still caught up on the Pochettino of seven years ago.
"He took Tottenham as far as he could, and then you've got someone like Rodgers who has done a far better job than what Pochettino did."