Another Lions Test, another epic of agonising drama. The series is in the balance, which is great for the series, but who knows what awaits us next? You feel the momentum has shifted in Australia's favour after those incredible final minutes, but will that change if James Horwill is unavailable?

And, just as importantly, will Jamie Roberts and Alex Corbisiero be fit? Because on these two hinge the Lions' selection and tactical policies. Roberts is the catalyst for everything. Without his punch through the middle, his ability to breach the gainline, the Lions' persistence with their current strategy will not work.

I have to focus this weekend on the scrum. I think Warren Gatland made an error in picking Mako Vunipola and I cannot believe he left him out there for the entire match. Australia, again, were spot on tactically at the set piece, in this case the scrum.

Ben Alexander, Stephen Moore and Benn Robinson all hit across to the left. Normally, this would leave Alexander, the tighthead, vulnerable to Vunipola coming across underneath him, but Vunipola stayed square for 30 minutes, which left him stranded alone, and you could see him falling out the side of the scrum. It took 30 minutes before he managed to come across to take Alexander and give the Lions a decent scrum. Vunipola can't get his hips low enough in relation to his shoulders to be able to attack Alexander, so he ends up scrummaging high, which makes it really obvious when he's coming across. He ends up getting penalised, either for that or just for being left out on the side.

Australia did the same to Joe Marler in the autumn international against England. As soon as they hit left it forces the loosehead's hips to come out, and then the Aussies can go into the space between the loosehead and the hooker.

The Lions' next problem is that Tom Youngs is not big enough to keep them out. So he and Adam Jones end up with enormous pressure coming through them, and Vunipola gets blown out the side. In that context, I'm astounded, firstly, that it took 56 minutes before Richard Hibbard, a much stronger scrummager, came on to block Alexander out. The Lions' scrum changed dramatically on their own ball when he did. And, secondly, that Ryan Grant never came on. Grant is technically a better scrummager than Vunipola. I coached him in the last Six Nations with Scotland, and I felt he should have been picked for the tour in the first place. But he suffers from this perception – inaccurate, I might add – that the Scotland scrum is weak.

I just don't understand the rationale behind leaving Vunipola out there, especially when you effectively can have rolling substitutions for the front row. Players can come back on if someone is injured there, so there was nothing to stop him doing it. But Gatland clearly has no confidence in Grant, which is a shame and, more than that, a serious problem if Corbisiero is not fit next Saturday.

The combination of a compromised set piece and the lack of a physical weapon in midfield to hit up over the gainline is killing the Lions. This is why Roberts and Corbisiero are so crucial to next week if Gatland wants to persist with the current gameplan. Otherwise, I'm not quite sure what the answer is.

The lineout was a little more ambitious, but there were still problems winning good ball at the tail. They drove the lineout very well in the first 20 minutes, so it's far from the end of the world, but the Gatland ploy of big runners coming round the corner off the back of a threatening set piece is being systematically undone by the Aussies.

Roberts and Corbisiero could go a long way to alleviating that. And I think Hibbard's greater presence at scrum time could as well, certainly if Corbisiero doesn't make it. The Lions need to rediscover life on the front foot, because Australia are defending with 13 men in the first line and Kurtley Beale and Will Genia marking the back on either side of the field. If Jonny Sexton can get the offensive kicking game going that we know so well with Leinster, there is lots of space just behind the first line of defence. But he needs to be on the front foot.

That's why Horwill's hearing on Monday could prove series-defining. He is the charismatic heartbeat of this Wallabies team. If he is banned for the decider, I'm not sure Australia will be the same team. For what it's worth, I don't believe he brought his boot down on Alun Wyn Jones's head on purpose – his immaculate disciplinary record and the fact that his view of Jones's head is obviously blocked tell me that – but there is a case to answer for reckless use of the boot. He may yet miss the final Test.

And that may help the Lions. One thing I am sure of, though, is that carrying on as the Lions are, without the thrust of Roberts and/or the threat of a more confident set piece, would be madness. Australia are working them out.