RB: RAFAEL IS LIKE A NEW SIGNING. Otherwise, I don't think LVG's 4-3-3 requires a particularly unique RB. Just a talented one who can combine with Mata and Herrera and defend a high line. Danilo has looked very good, but he's sadly off to Real and I doubt they'll let Carvajal go this summer. Maybe Carvajal will lose the battle and force his way out and PSG or someone else will pay 30 million for him next summer or in 2017, but I don't think he's going anywhere yet.
CB: Passing and ability to step up for LVG. For United's squad, we need an organizer and probably someone to play LCB since Smalling, so Hummels makes a lot of sense, but so would Godin, Thiago Silva (maybe in a Di Maria swap?) or if we're shopping a bit cheaper, Alderweild makes some sense, as would Abdennour in that he's been a rock for Monaco in the CL, is left-footed and his backline has looked organized no matter who the RCB is. Gimenez surely isn't available and is too inexperienced, but he's so solid whenever he plays that I'd be happy with him instead.
CM: Anyone who can link play as a CM, press and has the quickness or power to get wide when we fan out after we win the ball. Gundogan could do it but Koke would be a perfect fit. Matuidi would be brilliant as the LCM, too. Vidal a great fit on either side. Pogba has all the skills, though I think he might need some time to get on LVG's page.
As for replacing Carrick, I'd wait until the summer of 2016, at which point Carrick will either be leaving to go to QPR or whatever, or more of a depth player anyways, coming in for 20-30 minutes and getting the occasional start, filling in for injuries and providing experience and stability around younger players in Cup games.
Winger: A goalscoring winger makes sense and may well arrive in Depay when you consider that Rooney is our only major goal threat and Mata would be the 3rd or 4th highest scoring type on a team like Madrid, Bayern or Barca (assuming he started) rather than our clear number 2. Bale would be ideal, even if he had to play on the left and cross a bit more, hopefully having something like a 20 goal, 15 assists (in all Comps) season rather than the 30G + 10A sort of season you might get expect from him on the right.
Brahimi's not a great goalscorer but he's good at everything, doesn't lose the ball and works hard. I think he'd please LVG.
I'm not sure we absolutely need a winger if LVG really rates Young, as he and Di Maria can fight out left and Mata can be the RW, backed up by Januzaj. If we do sign one and Di Maria stays and LVG thinks this Young thing will continue for another year, I'd loan Januzaj somewhere for a year.
TOP CHOICES:
CB: Hummels/Godin/Silva
RB: Whoever we think is the best RB out there (no major style issues)
CM: Koke/Vidal/Matuidi/Pogba/Gundogan
W: Bale/Depay/Any Winger who can score goals from the left-wing, combine with teammates and work hard.
IF WE HAVE LESS MONEY/STRIKE OUT EVERYWHERE/INTERESTING TO THINK ABOUT
CB: Abdennour/Alderweild
RB: Trippier has impressed me a lot. I sort of hope we sign someone young and promising and cheap-ish like him and give Rafael first shot.
CM: Hard to think of a physical CM with the technical quality needed who wouldn't cost a ton.
W: Hermann doesn't give the ball away easily and has added some goals this year.