One got the feeling she, for whatever reasons, was particularly afraid of getting into foul trouble, so her defense was sometimes timid. If she accepts that she is a role player who gets, at most, 8-10 minutes per game, she may come to realize that she can afford to play stingier defense in that capacity.
Mercedes is unlikely to ever have the weight of the team on her shoulders and, so, will be freer to commit a few "message fouls" that she may never have felt comfortable committing before for fear of fouling out.
I've played with a player or two back in the day who were deathly afraid of fouling out and it hurt their game. Maybe Mercedes was one of those players. I mean, did she ever foul out of a game at Tennessee? And if not, doesn't that seem a bit strange for a center?
Russell should just go for it. Let her size work as an intimidation factor. But she'll have to put the fear in opponents -- like Griner used to do to our Lady Vols. :ermm: