Relief Pitcher Disabled List Chances

A few years ago I broke down the disabled list (DL) chances for starting pitchers (39%). I have finally gotten around to making an initial stab at relief pitcher DL values.

The initial problem I have with relief pitchers is defining them. I didn’t want to be looking at pitchers who split their time between a relief and starting role. I decided the pitchers needed to have at least 95% of their appearances as a reliever. Also, I didn’t know what to set the minimum innings limit to. Here is a chart of the DL% depending on the number of innings pitched the previous season. Matched season data from 2002-2003 to 2012-2013 was used.

IP	Pitchers	Pitchers Who Went on DL	DL%	Total DL Trips	AVG Days per DL Trip	AVG Days lost per pitcher
20	2275	741	33%	876	58	22
30	2015	670	33%	798	56	22
40	1744	588	34%	708	55	22
50	1422	487	34%	592	53	22
60	996	337	34%	411	53	22
70	510	157	31%	188	52	19
80	164	43	26%	53	48	16

Not a huge difference in values until a pitcher reaches the 70 and 80 IP level. For all the rest of the analysis, I will use a 20 IP limit (33%) as a baseline. Basically one out of every three relief pitchers should expect to go on the DL for around 55 days. With the average bullpen being seven pitchers deep, the team will need to have two more pitchers who can fill in as the others get hurt (7 * 33% = 2.3 pitchers)

Additionally, it may be counter intuitive that the more innings a pitchers throws their DL chance drops. As I found with starters, if a pitcher has proven they can throw a bunch of innings once, they can probably do it again.

Now on to see about the age component. I looked at the DL chances and days lost with two year time intervals.

Age	Pitchers	Pitchers Who Went on DL	DL%	AVG Days per DL Trip
22-23	52	16	31%	65
24-25	214	61	29%	76
26-27	432	121	28%	66
28-29	454	133	29%	54
30-31	348	124	36%	53
32-33	278	104	37%	55
34-35	212	71	33%	64
36-37	147	53	36%	48
38-39	89	37	42%	42
40-41	35	14	40%	48

Some trends can kind of be seen. I went a little further and grouped the data into 5 year grouping where some dividing lines can be seen clearer.

Age	Pitchers	Pitchers Who Went on DL	DL%	Total DL Trips	AVG Days per DL Trip	AVG Days lost per pitcher
20-24	137	40	29%	46	70	24
25-29	1017	291	29%	338	62	21
30-34	741	269	36%	312	56	24
35-39	333	120	36%	151	49	22
> 39	47	21	45%	29	49	30

Two trends stick out now. Younger pitchers are less likely to go on the DL, but when they do, they are more likely to stay longer. The average number of days lost per pitcher (not DL trip) is relatively constant until a pitcher reach the big 40.

So age and previous innings pitched don’t really change a pitcher’s DL chances. Now on to an item which does, previous DL stints.

DL Status	Pitchers	Pitchers Who Went on DL	DL% year2	Total DL Trips	AVG Days per DL Trip	AVG Days lost per pitcher
DL year1	529	224	42%	308	64	37
No DL year1	1746	517	30%	568	55	18

So relief pitchers who went on the DL in previous season see a 43% increase in their DL chances and when they go on the DL it will be 16% longer compared to the baseline 20 IP values. This increase is important for a bullpen full of injured relievers (see the Rangers). If  a team has all seven of its relief pitchers on the DL in the previous year, it will need three pitchers lined up to fill in (42% * 7 = 2.9).

Well that is it for now. Next up, I will run a logistic regression against the pitcher data to come up with a formula to give each pitcher a DL. Until that date, at least a person now should have a basic understanding of a relief pitcher’s DL chance and the main factor which can increase it, previous injuries.