I think it's way too much to say there's "never" a silver lining to injuries and losses, at least in the regular season and particularly when the team has already wrapped up a playoff spot and is close to clinching the division. This is as ideal a time for this to happen as any - they only need six wins in their final 17 games to guarantee at least the third seed, and I think they handle that with or without KG. If the team is gonna hit a bump, it's better to have it happen with 30 games to go in the regular season while ahead of the pack, like this was, rather than with 5 games to go or after the playoffs start. Do we want the injuries? No, of course not. But it's only a silver lining (and not, say, a golden opportunity), precisely because it surrounds a cloud over the season, a bad situation. Only the lining is silver, not the situation as a whole.
I will agree, though, that there won't be any silver lining if Garnett can't come back healthy. So provided Garnett comes back before the playoffs, it's a good time for him to get a lot of rest - if he has more energy in the playoffs because of this rest, rehab and recuperation, he'll be even tougher, much like he started both last season and this season with a vengeance. And it's an opportunity for the bench, which has been frankly woeful this year on the whole, to sort out who the team can rely on come playoff time. On that front, the Allen, Davis and Scalabrine injuries hurt even more, but like the OP said, Moore and Marbury need to figure things out and now they have a trial by fire.
I don't agree with everything the OP said - specifically, I disagree that one game from Walker proves he'll bring it every night. I'm optimistic about him too, and would love to see him get more burn, but one game does not prove everything. It proves potential and opens the door to an opportunity to prove he will bring it every night. But his ability to contribute is not a foregone conclusion. I also am not sure about the Davis thing, especially the idea of him gaining confidence while on the bench. He definitely played adequately in Garnett's absence, which would have boosted his confidence, but if anything, his injury is pushing him further out of the rotation, as Powe is looking like he'll take all the backup PF minutes and Davis will be fighting Moore and maybe Scal for minutes in the playoffs. And I just don't see how he or anybody else gets in their head that if he was playing, they would have won either or both of the last two games. To reach that conclusion in games where two starters are missing is a bit much.
I agree completely, though, that the injuries has forced Marbury and Moore to see how far off they are from being ready to contribute to a good team. Hopefully it accelerates their pace of getting up to speed, which would most definitely be a silver lining. With Rondo out, Marbury saw how he would play in extended minutes with Pierce and Allen. And he and we saw it wasn't very good. He can move forward from that, and we get to see him in more situations to decide where he does and does not fit. And if neither guy takes the opportunity to push forward in getting into the system, it won't be because of the injuries, it will be because those guys just don't have what we need. Silver lining - we find out sooner rather than later, when we erroneously rely on them in the playoffs.
I also agree that Powe has taken advantage of the opportunity and HIS confidence is rising. He can play with the starters, he can play with the bench players, he knows how to fit in both situations. I think he'll be crucial especially if Garnett needs a little extra rest in the playoffs, and this opportunity has picked him back up. He had an inconsistent at best season, but he's coming on strong now at the right time. If Garnett was healthy and Scalabrine was healthy and Davis was healthy, who knows how often Doc would have called Powe's number. Lord knows he didn't do a ton this season to merit inclusion as one of our key bench players. Without the injuries, maybe Scal and Moore (to get him involved) and Davis are splitting all the minutes at the big positions. Instead, Powe got the call and has run with it.
I also agree that the overall confidence of the team has to be rising a bit since they are at least competing and playing very well against other playoff contenders despite missing two key starters - one an All Star and the defensive catalyst of the team, the other a strong All Star candidate, key perimeter defender and now the offensive catalyst as point guard - and three other rotation players. Ideally, they would have won at least one of the last two to go with the great Cleveland win (had they gone 3-0, I think you could have drawn a lot of parallels between that three game stretch and the Texas Triangle last year, although we had a lot more adversity in this stretch), but staging those comebacks was huge even if they fell short. We all know that when healthy, this team comes out strong to start halves due to their great starting five. They probably wouldn't be in that come-from-behind position they've been in the past couple of nights if they had Rondo and KG there.
So there are definitely silver linings. That doesn't mean the situation was ideal or a blessing in disguise (not the same as a silver lining) but good can come out of it, and from where I'm sitting, the Celtics are getting some good things out of a bad situation.
