The England international made the move to the Catalan giants on loan in the summer after being part of United manager Ruben Amorim's “Bomb Squad” this summer.
Rashford has found his stride since the September international break and has been one of the best players in the La Liga side this season despite his many critics.
He has either scored or assisted in each of his last six matches for Barca under manager Hansi Flick who has the option to keep him for just £30M at the end of the season.
Rashford picked up another Barcelona assist in their game against Paris Saint-Germain this week in another impressive performance. However, speaking on Sky Sports, Sherwood joked that Rashford has been below par.
"He's (Rashford) done OK. It suits him better when they had more control and they're getting the ball out to him. But it's unfair for him because Lamine Yamal gets it on the right-hand side, does a lot of pirouettes, the crowd is up, he has shots at goal, and they drop off him.
"When it goes out to the left to Marcus, he is one dimensional, we know what Marcus is and he's very effective when he does it. He knocks it, he runs past people and when he doesn't the crowd think, 'why don't you give us the same as the genius on the other side?'"
"It's harsh on him I know, but it's a very difficult environment for him to play in. Marcus Rashford will develop if he wants to and get better playing in this Barcelona side. He has to get better."
"At the moment, he looks bang average because the kid on the other side is so good."
Despite Sherwood’s comments, Flick has expressed satisfaction with the winger's adjustment after arriving from the Premier League.
"Everyone could see it in the last games. In Newcastle he gained a lot of confidence. It's not always easy, he came from another league. He is adapting well."