Manchester United were beaten 2-1 by Pep Guardiola’s men, who equalled the Premier League record of 14 for consecutive wins.
It was a damaging defeat for United and manager Jose Mourinho, who came in for criticism for his tactical approach.
And Merson has given a scathing assessment of United’s display, claiming he was embarrassed by them.
He told Sky Sports: “I thought Manchester United were the only team that could beat Manchester City but I was embarrassed by them last week, I thought they were atrocious.
“A big team like Manchester United playing at home and just trying to keep the score down. It was really poor.”
United travel to West Brom on Sunday on the back of a narrow 1-0 win over Bournemouth on Wednesday.
The Baggies earned a hard-fought 0–0 draw with Liverpool but have struggled to score goals all season.
New manager Alan Pardew has been unable to make an immediate impact on his side’s goalscoring problems.
And Merson reckons West Brom’s woes will continue on Sunday.
He added: “Are West Brom going to score a goal?
“It’s alright changing managers and saying we are going to do this and that, but what’s changed? Nothing.
“Manchester United haven’t been great, and they were poor again against Bournemouth in midweek, but they will have too much for them.”