Let's me try a couple of things.
First, the bull pen. The first stat is
Wlst (Wins Lost). This is the number of Wins lost to a starter who exited the game with the lead (chance to get the win) and the game was lost by the bull pen.
2011 = 5, 1st (lowest in the AL and 6 less than the next team, 14 was AL average)
2012 = 11, 6th (12 was AL average)
2013 = 13, t4th (15 was AL average)
2014 = 15, t8th (14 was AL average)
2015 = 14, t7th (14 was AL average)
The bull pen has been "average" at giving up wins over the last 2 years and "above average) over the last 5. If 14 has been the AL Average over the last 5 years and DET has averaged 14 over the last 3, how exactly have they been costing the Tigers wins if this is the case?
Next would be the next stat,
Lsv (Losses Saved), this would be the games where the starter exited the game with a loss at hand, but the team came back to tie or take the lead.
2011 = 22, 2nd (18 was AL average)
2012 = 24, 1st (17 was AL average)
2013 = 16, t9th (19 was AL average)
2014 = 16, t8th (17 was AL average)
2015 = 17, 9th (18 was AL average)
The AL average has been 18 over the last 5 years and DET has average a little more than 16. But this stat isn't just about the bull pen. It also has the element of the offense coming back and scoring runs.
Offense Runs 7-9 Innings
2011 = 218, 4th (201 was AL average)
2012 = 194, t10th (201 was AL average)
2013 = 179, t12th (193 was AL average)
2014 = 231, 3rd (198 was AL average)
2015 = 190, t12th (204 was AL average)
DET's late game offense has been well below average the last 5 years or so. If you take this into account and what is actually the result of the bull pen, any change "might" mean 1-2 wins difference. Until they actually play some games, we don't even know if they even improved.
Finally, the starters. Record for starters, Quality Starts and Run Support. Given league average Run Support and league average bull pen, a team should win approximately as many games as they have Quality Starts.
2011 (95 wins) = 72-47 .605 WPCT 90 QS (t6th, 86 AL avg) 5.0 RS (4th, 4.5 AL avg)
2012 (88 wins) = 63-51 .553 WPCT 90 QS (t2nd, 81 AL avg) 4.6 RS (5th, 4.5 AL avg)
2013 (93 wins) = 76-44 .633 WPCT 108 QS (1st, 83 AL avg) 4.9 RS (2nd, 4.3 AL avg)
2014 (90 wins) = 68-55 .553 WPCT 90 QS (3rd, 83 AL avg) 4.7 RS (1st, 4.2 AL avg)
2015 (74 wins) = 47-66 .416 WPCT 72 QS (t11th, 81 AL avg) 4.3 RS (8th, 4.4 AL avg)
Number of starters with at least 29 Games Started
2011 = 4 (129 GS, or 32.3 GS avg)
2012 = 3 (96 GS, or 32.0 GS avg)
2013 = 5 (156 GS, or 31.2 GS avg)
2014 = 3 (96 GS, or 32.0 GS avg)
2015 = 1 (31 GS, or 31.0 GS avg)
2013 Starter's Career QS % through 2013
J. Verlander 266 GS 171 QS = .643
D. Fister 127 GS 77 QS = .606
A. Sanchez 173 GS 103 QS = .595
M. Scherzer 165 GS 96 QS = .582
R. Porcello 149 GS 74 QS = .497
2015 Starter's career QS with at least 12 GS in career
D. Price 213 GS 147 QS = .690
J. Verlander 318 GS 203 QS = .638
A. Sanchez 219 GS 124 QS = .566 (46 GS 21 QS .457 last 2 years)
R. Wolf 379 GS 205 QS = .541 (37 GS 12 QS .324 since 2011)
A. Simon 82 GS 42 QS = .512 (50 GS 20 QS .400 not including 2014 Career year)
S. Greene 30 GS 11 QS = .367
K. Lobstein 17 GS 6 QS = .353
M. Boyd 12 GS 4 QS = .333
D. Norris 14 GS 3 QS = .214
2016 Projected Starter's career QS with at least 12 GS in career
J. Zimmermann 178 GS 115 QS = .646 (153 GS 107 QS .699 last 5 years)
J. Verlander 318 GS 203 QS = .638
A. Sanchez 219 GS 124 QS = .566
M. Pelfrey 213 GS 98 QS = .460 (100 GS 40 QS .400)
S. Greene 30 GS 11 QS = .367
M. Boyd 12 GS 4 QS = .333
D. Norris 14 GS 3 QS = .214
Last 5 years
D. Price 158 GS 112 QS .709
J. Verlander 153 GS 107 QS .699
J. Zimmermann 153 GS 107 QS .699
D. Fister 129 GS84 QS .651
M. Scherzer 163 GS 106 QS .650
A. Sanchez 138 GS 82 QS .594
R. Porcello 150 GS 86 QS .573
M. Pelfrey 100 GS 40 QS .400
S. Greene 30 GS 11 QS = .367
M. Boyd 12 GS 4 QS = .333
D. Norris 14 GS 3 QS = .214
Since 2013, we have regressed in the ability to have Quality Starts. This impacts the team in multiple forms. Quality Starts means the starters are keeping us in the games and less need to access the bull pen early. We all know a tired bull pen is less effective. Pelfrey + D. Norris/S. Greene/M. Boyd are at least 12-14 QS less than Porcello/Fister combo. I don't see that improving much. Heck, by himself, Pelfrey is at least 5 less QS (wins) by himself.
I really haven't gone into the offense and how it really hasn't improved.
Here is a link about age and injuries:
http://makenolittleplans.net/age-position-and-injury-risk-in-baseball/