Bullpen Fatigue and Availability: What the Data Says
Starters now record about 15 outs a game, which leaves roughly four innings to the bullpen every night. Which relievers are available for those innings changes daily, and managers are predictable about it.
The availability rules
- Threw 25 or more pitches yesterday: unavailable.
- Pitched on each of the last two days: unavailable.
- Pitched three of the last four days: unavailable.
- Pitched yesterday but under 25 pitches: half available.
- Otherwise: available.
Each reliever is weighted by how much he has pitched in the last three weeks — the closer and top set-up men carry the most weight — so losing them matters more than losing a mop-up arm. The bullpen tables on every game page and team page show the last three days of pitch counts and flag who is likely down.
How much does it matter?
Less than most people expect. In our backtest, once the starter's expected innings and the quality of the relievers who are rested are known, the extra 'total pitches in the last three days' signal moved expected runs by well under 1%. The reason is simple: teams manage fatigue. When the top arms are down, the replacements are worse, and the model already captures that through the quality of the pitchers who remain. What matters most is how good the available arms are and how many innings they will need to cover.
In the postseason
October changes the pattern: off-days mean fresher bullpens, starters are pulled earlier, and the best relievers pitch more often. The same rules still apply day by day, which is why the model is run through the playoffs without changes.