Firstly, the words "Fred" and "ball retention" should never be uttered together.
As for McTominay, he was poor today. Carried the ball too much when he shouldve passed, and was sloppy in possession. In terms of "serving a purpose once the team is settled" however, McTominay could very well prove valuable as an attacking #8. He is normally good on the ball, at beating the press and carrying it out on the turn, and pinging balls into the feet of the attackers. He is never going to be a playmaker, and he is never going to be a DM as his defensive awareness and position are his two worst attributes. He also adds some physical presence to the team, which can be useful when we have signed a short CB.