Those runs given up by McCann's catching is more of a product of the bullpen and starters being absolute shit. Avila's had the good fortune of catching behind JV in his prime, Scherzer, the good version of Sanchez, Fister, etc... Who's McCann got other than one half-season of David Price?
Too bad the results do not back up your statement.
Ian Krol
w/Avila 2.59 ERA 1.34 WHIP .250 BAVG .325 OBP .417 SLG .742 OPS
w/McCann
10.03 ERA 2.57 WHIP .382 BAVG .477 OBP .582 SLG 1.059 OPS
Tom Gorzelanny
w/Avila 4.15 ERA 1.85 WHIP .333 BAVG .400 OBP .444 SLG .844 OPS
w/McCann
8.44 ERA 1.81 WHIP .344 BAVG .411 OBP
.585 SLG .996 OPS
Blaine Hardy
w/McCann 2.25 ERA 1.11 WHIP .220 BAVG .281 OBP .300 SLG .581 OPS
w/Avila
3.38 ERA 1.31 WHIP .220 BAVG .304 OBP .317 SLG
.621 OPS
With Starters
David Price
w/Avila 2.44 ERA 1.08 WHIP .218 BAVG .272 OBP .302 SLG .574 OPS
w/McCann 2.57 ERA 1.12 WHIP
.253 BAVG .285 OBP
.409 SLG .694 OPS
Anibal Sanchez
w/Avila 3.96 ERA 1.09 WHIP .214 BAVG .271 OBP .328 SLG .599 OPS
w/McCann
4.80 ERA 1.37 WHIP
.266 BAVG .326 OBP .465 SLG .791 OPS
Justin Verlander
w/Avila 4.19 ERA 1.14 WHIP .270 BAVG .289 OBP .378 SLG .668 OPS
w/McCann 4.14 ERA 1.24 WHIP .243 BAVG .304 OBP .400 SLG .704 OPS
Kyle Lobstein
w/Avila 2.14 ERA 1.19 WHIP .234 BAVG .298 OBP .286 SLG .583 OPS
w/McCann
6.08 ERA 1.65 WHIP .318 BAVG .367 OBP .482 SLG .848 OPS
Alfredo Simon
w/Avila 4.58 ERA 1.58 WHIP .300 BAVG .364 OBP .414 SLG .778 OPS
w/McCann
5.01 ERA 1.44 WHIP .276 BAVG .333 OBP
.471 SLG .804 OPS
Shane Greene
w/Avila
7.03 ERA 1.44 WHIP .283 BAVG .346 OBP .475 SLG .821 OPS
w/McCann 6.65 ERA 1.56 WHIP .314 BAVG .371 OBP
.562 SLG .933 OPS
Closer
Joakim Soria
w/Avila
3.72 ERA 0.62 WHIP .152 BAVG .200 OBP .242 SLG .442 OPS
w/McCann 2.36 ERA 1.07 WHIP
.198 BAVG .274 OBP .375 SLG .649 OPS
**BOLD = 35 point difference or greater
The take away is that the vast majority of DET pitchers have a significantly higher SLG Against when McCann is catching. Why do you think there is that difference?
And you cannot just discount everything based on a player being a rookie. There have been plenty of rookies in MLB history that had their career year as a rookie.