80 10 21 11 10 10 81 51 41 (755)
20 10 20 50 81 12 02 80 (714)
32 90 21 21 90 50 31 10 52 91 (660)
60 81 10 41 11 81 51 02 90 41 91 51 41 (586)
80 10 81 31 51 41 11 90 21 21 50 20 81 50 32 (573)

(Note: The numbers in parenthesis are not part of the code; rather, they refer to the "ballpark figure" that precedes them.)

With some discrepancies found, the code seems to refer rather to Harmon Killebrew as the last player, rather than Don Mattingly.

80 is H from the first letter in HankAaron
10 is A from the second letter in HankAaron
81 is R from the fifth letter in BabeRuth (though Barry Bonds' R's are 21's)
31 is an O according to the seventh letter in BarryBonds, but this might be a mistake.
51 is also an O, from the next to last letter in HankAaron
41 is an N from the last letter in HankAaron, though a 21 is used for the first N in that name.
11 is a K from the fourth letter in HankAaron
90 is the I in FrankRobinson (though it's also the A in Barry Bonds)
21 is an N in HankAaron, but this might be another mistake.
21 ditto
50 is the E in BabeRuth (but also the second B in BarryBonds)
20 is both B's in BabeRuth, though 32 serves as the first B in BarryBonds and 50 the second.
81 is an R again as in BabeRuth
50 is an E again as in BabeRuth
32 is a W this time.