“Don Mattingly’s career was too short, but few would say McGriff was as good a player as Mattingly at their best. I wouldn’t.”
From 1984-86, Mattingly had an OPS+ of 158. From 1988-90, McGriff had an OPS+ of 159. McGriff’s 1988-91 OPS+ of 156 also beats Mattingly’s 1984-87 OPS+ of 155.
But in WAR, Mattingly is ahead over three years 19.6-16 and ahead over 4 years 25.3-20.6. Over the 4 years Mattingly had 0.7 more defensive WAR, so that is only a small part of the difference, meaning Mattingly’s offensive WAR advantage was 4.0. He had about 157 more PAs. It does not seem like that would give him an edge of 4.0. I can’t tell what accounts for it.
I also found Mattingly’s best 4 year period for Win Shares was 14 better than McGriff’s (122-108). Mattingly had 2.3 more fielding Win Shares, so most of the difference is due to hitting. Again, when their OPS+ is so close, this is surprising.