A major league team filled with a roster of "replacement level players" would be expected only to win 48 games. In reviewing the Cubs 2018 season, let us look at a few key statistics.
The Cubs won 95 games. That is 47 games above replacement level.
The Cubs combined hitters had a 24.0 WAR.
The Cubs pitching staff had a 21.1 WAR.
Combined, the roster had a 45.1 WAR.
In the end, the Cubs wins was 1.9 above their collective WAR. So the Cubs won two more games than reasonably expected during the season. Is this the quantitative measure of Maddon's managerial skill?
If you look at the Cubs win total to actual WAR, the 1.9 WAR difference is 4.2 percent. That would equate to a projected seasonal impact in approximately 7 games (6.82).
The Cubs were 11th in the NL with 104 errors made. That means the Cubs gave their opponents 104 more outs, or the equivalent of 3.85 games. For a tired pitching staff, adding another 4 games to 163 regular season contests is a burden. They had to perform 2.4 percent more than the norm.
Errors create base runners which leads to more "highly leveraged" pitching situations. For starters, that means changing one's mechanic's to the stretch position. For relievers, it puts more pressure to get strikes with your best pitch. For relievers, the errors could have added an average hidden 4 1/3 IP to their work load, or 4 more appearances during the season. Cishek had 80 appearances for 70.1 IP. Wilson had 60 appearances for 52.1 IP.
The Cubs season could be summed up as being conflicted; there was inconsistent offense and tired pitching but it lead to a slight overachievement in the end.