The Argentine started for the first time this season in Thursday night's 1-0 Europa League defeat to Feyenoord.
Rojo was used ahead of Luke Shaw, who stayed in England to freshen up for the weekend's clash with Watford, and former Red Devils star Scholes feels the 26-year-old performed well below the standards required.
"Rojo is a bit out of form," Scholes said on BT Sport.
"He's had a tough night, you can tell he's a player who hasn't played minutes this season. He's really struggled.
"If I'm a Feyenoord player and I'm looking at Man United he's a big weakness.
"If I'm getting the ball I'm just clipping it over his head over again and making him run."
The only goal of the game was scored by Tonny Vilhena in the 79th minute as United opened their European campaign with a disappointing result in Rotterdam.