You certainly make a good case and I agree that neither Scholes nor Giggs are going to be able to play without someone like Carrick partnering them - although I think Cleverley and Anderson could both surprise a lot of people defensively next season.
Every single big game we played last season we were overrun in midfield, as well as against a lot of supposedly weaker teams. As soon as anyone put a bit of pressure on us we crumpled like a deck of cards. Considering this, we still did extremely well it has to be said.
The problem then, isnt needing another player to screen the back four. It's having players in the middle of the park who are comfortable on the ball and can keep and dictate play when needed, any of the Spain midfield would do, based on tonight.
That as I see it is the problem, and the answer to that is a player like Modric, who (probably exaggerating) would single handedly transform our midfield in the bigger games. We have the likes of Rooney and Valencia providing additional defensive support as well as Carrick, and a world class defence. Defensive mindedness is NOT our problem in my eyes.
Again on the fitness issue, no we cannot guarantee that Carrick won't get a long term injury, but it's about probability and that is just a risk we will have to take unless we are prepared to sign two midfielders. We winged it last season with our current set of midfielders despite being outplayed in a lot of our games, hence why we failed so badly in Europe. The same sort of thing will happen again next season unless we address the real issue which is on how to respond when teams put a bit of pressure on us, because longer it goes on, the more teams will realise how easy it is to exploit that weakness.
But we weren't always over run and in the games that we were it wasn't with our first choice midfield. Against Ajax Carrick didn't play, against Athletic he only played the second leg. In the group stage we missed Carrick in a few games, particularly the deciding one, not to mention other injuries in the defence. On top of that in most games and really all season seeing he was never at his best we missed one of our first names on the team sheet for these big games- Fletcher. He would have come in with Carrick and given us a solid shield. I don't think we would have had any of the issues we had to the extent we did with both he and Carrick together for the bigger games. But he wasn't and due to his condition we can't rely on him to be back.
In europe we missed Carrick for a number of games as well as other problems and Fletcher when he was around wasn't at his best, clearly due to his illness.
But that's why we have two issues in the midfield. Firstly we lack another midfielder in their physical peak who is reliable fitness wise. Fletcher obviously has his illness, clev and ando have injury problems. Having another player like that around for most the season would help carrick a lot. They might not be as defensively good as him but they have the energy to help out and alleviate the burden on Carrick. Second half we had to rely on Scholes as everyone else was injured, a player who we stopped playing in the bigger games because he needed too much protection. Carrick can only do so much. No one can provide full cover.
Secondly without Fletcher we don't have a back up for Carrick or a defensively solid option to come in alongside him. This is big issue. Like I said Carrick can't always be expected to play, he didn't last season and we generally suffered without him.
Modric can help in the first situation but he can't in the second and that's why for me he's not a priority. If we can get him and another then great, but right now another player with defensive strengths, not necessarily a dm, is more important.
Looking at the spanish example you gave, their central 3 are very good defensively. Busquets and Alonso are as strong defensively as Carrick is, and I'm sure most would say they're better defensively. Both are comfortably better defensively then any of our other options and Modric. Xavi is better defensively than any of our midfielders bar Carrick. Scholes might have the edge in the final third but Xavi definitely has it from a deeper more defensive role. And that's regardless of their possession abilities. I'm talking purely about tracking players, tackling, positioning etc.
In time I would expect Ando and Clev to improve defensively, but they've got plenty to learn attacking wise let alone defensively and they still need to sort out their fitness issues. But we can give them another season for that. As I've said signing someone who's more defensively strong will give us competition/back up for Carrick as well as an option if they do get injured or if we want to be more solid. If we can etc Modric as well then I'd never say no, Fergie can have the issue of keeping them all happy, but just like we don't need another winger, or centre back or striker, we don't really need another creative player. We have players there even if we're unsure on how good they'll be. What we do need though is an alternative to Carrick.
Also I think getting in a partner for Carrick who doesn't need his protection would allow Carrick to venture forward more often. With Carrick getting his form back for a solid year now I'd expect it to continue and I think he showed last season that when he does get forward he has something to offer. A reliable partner would let him show that. But even without that Carrick and another solid partner would give the others a platform ahead to work off and would mean the likes of Rooney and Young don't have to track back loads to hep cover.
Like I said if we could get that sort of player and Modric then perfect, but if it's just one then I think we need that sort of player far more.