The Dutchman was sacked by United earlier this season, with McCarthy working with Ten Hag at United during his first two seasons at Old Trafford.
Ten Hag, who left McCarthy go this past summer, is said to have favored his own signings when selecting teams.
Speaking on the Podcast and Chill Network, McCarthy was asked why Amad Diallo had not played more.
He stated: "Tactically, when we set up, the manager felt that Antony was the best, better player for the job that he wanted because he knew Antony from Ajax. He was his player, so he invested in him, and the club supported him.
"So, of course, he had to support the player that he bought, I think that's why he persisted in playing Antony. It's because he knew the capabilities when Antony was at his best.
"And then you had (Alejandro) Garnacho who was also playing well, scoring goals. (Marcus) Rashford, on any given day, could demolish anyone. And then you've got (Rasmus) Hojlund, you spend £70m on bringing him in, so he has to play.
"So someone's gotta (miss out), and unfortunately for Amad, the manager goes with the player he thinks is probably more capable for, for those type of games tactically, defensively and attacking."