On average, a Manchester United player has played 1.6 more games (150mins) more than the average Liverpool player (from all players who have played minutes this season, all competitions).
The average minutes played by the top 11 Liverpool players (by minutes) is 2,393 (26.6 games) while the top 11 for Manchester United is only 2,181 mins (24.2 games). The top players are playing more
But I think a key stat is that in terms of minutes played, those in positions 21, 22, 23 (fringe squad positions) are Pogba, P Jones and Chong for United, and C Jones, Kelleher and WIlliams for Liverpool.
Haven't got other teams data, but I would think that City's would also be interesting