The Red Devils recorded a 2-0 win against the Baggies, with Lingard opening the scoring before Juan Mata wrapped up the points by scoring a penalty in injury time.
Ex-Arsenal ace Merson told SkySports he was particularly impressed with Lingard's direct running, but felt Smalling's display at the back was even more important for the Red Devils.
He said: "I thought he (Lingard) was one of the better ones. When he got the ball out wide he went, bang straight down the line, he'd get a throw on, or corner. I thought he was positive.
"I thought he done well but Smalling was absolutely outstanding, he's getting better and better every week."
Chris Smalling has blossomed this season into the player Manchester United always hoped he would be.
Manager Louis van Gaal has taken a lot of criticism of late for the lack of flair in the side, but that is a price they pay for his defensive approach, which has enabled Smalling to become one of the best centre-backs in the Premier League.
A year ago he was a punchline after getting sent off at The Etihad in the Manchester derby, but right now he is a player United cannot afford to do without.
Lingard also making good on his much lauded potential, having been robbed of the chance to do so last season by injury. It would have been easy for the club to ship him out on loan this season, but he is keeping Memphis Depay out of the first team, and growing in confidence with every appearance.