Lee would, under normal circumstances, recieve a few more matches grace before his position came under question because of his performances last summer. Australia will usually look to drop older players when there's a clear chance and they aren't playing well, but Lee is only 30 and is seen as part of the attack in the post-McGrath era, and he did lead the attack for the period McGrath was away to justify those hope somewhat, so they won't be keen to drop him.
The problem is, if Perth is another road which turns a little and Watson is fit they'll be looking to pick MacGill, which means a seamer has to go. Clark is more or less undroppable at the moment as he's the form bowler in the side, and McGrath is McGrath. It's extremely harsh on Lee, but he may find himself on the outer for the third test, unless he bowls well in the second innings here.
Or hell, they might leave MacGill out again. Hard to say. Either way there's one seamer and one batsman who may be on the way out after this test, so nobody will want to fail.
To answer the McGrath question, I'm not sure it means that much. It was a fairly poor performance and not something we would have seen from McGrath too many times in the past, but he did actually bowl pretty well at times and was the only bowler who consistently troubled Collingwood. I view it similarly to Warne really, who has bowled fairly poorly at times in this series, but the workload and the batting hasn't exactly helped, and he's also been extremely good at times, like in the first two sessions on day one in Adelaide. I think it'd be sensible to reserve judgement on McGrath until the series is over.