We seem to have a lot of discussion of possible changes to the Claw / PO / MB rules... don't really want to get into whether any of these could or should be implemented, thought it might be nice though to see how the maths works out for some of 'em.
In each case, the probability includes the need to knock the player down, and all odds are for a Claw / PO / MB attacker. I'm assuming 2 dice, ignoring the effects of team RRs, and ignoring other modifying skills like Stunty, Thick Skull etc. Odds are for any off-the-pitch injury, and I assume the attacker will use their skills to maximise the chance of this (e.g. no rerolling a KO result).

First off then, the combo as per the latest rules.

Defender skills Probability
(None) 43.9%
Block / Wrestle 32.5%
Fend 31.8%
Block / Wrestle and Fend 13.9%

Now nerf #1 - PO only works if you follow up. Note Stand Firm now affects the odds, in the same way as Fend.

Defender skills Probability
(None) 41.9%
Block / Wrestle 32.5%
Fend / Stand Firm 23.5%
Block / Wrestle and Fend / SF 13.9%

Nerf #2 - PO lets you reroll armour only, i.e. back to the LRB4 rule.

Defender skills Probability
(None) 33.2%
Block / Wrestle 24.6%
Fend 27.4%
Block / Wrestle and Fend 13.9%

Nerf #3 - PO doesn't stack with MB, if you reroll armour or injury, no +1 modifier.

Defender skills Probability
(None) 39.5%
Block / Wrestle 29.3%
Fend 30.0%
Block / Wrestle and Fend 13.9%

Nerf #4 - PO doesn't stack with MB or Claw. Note that we now get different odds for different AV defenders.

Defender skills + AV Probability
AV7, no skills 39.5%
AV8, no skills 37.0%
AV9, no skills 34.9%
AV7, Block / Wrestle 29.3%
AV8, Block / Wrestle 27.4%
AV9, Block / Wrestle 25.9%
AV7, Fend 30.0%
AV8, Fend 29.0%
AV9, Fend 28.1%
Any AV, Block / Wrestle + Fend 13.9%

I'm sure there are mistakes, feel free to point 'em out. Or suggest other nerfs if you like :)