This. If we are going to get another ball playing defender it means we are going to stick with 3-5-2, however with the signing of Di Maria, I am convinced we are moving towards a 4-3-3 which is LVG's preferred formation. I think one of Evans/Jones/Smalling will thrive alongside a more imposing and commanding defender who is also good with the ball at his feet. That defender could be Rojo, he is as tough as nails and commanding whilst he is good with the ball as well as shown be his proficient displays at left back. He would perfect to partner one of our existing defenders and I think it would be good enough.
The main problem is lack of cover/protection for our centre backs from the midfield. Fletcher and Cleverley have been gash in this regard and offer next to no protection when it comes to screening the defence or acting as a ball terrier by vigourously closing down the opposition whilst putting a fair amount of tackles in. We could ideally do with a commanding ball-winning presence in front of the midfield ala Vidal but with Carrick, Herrera and Di Maria in midfield this would be very good. Carrick screens in front of the defence with his excellent positioning and reading ability while Herrera and Di Maria(to a lesser extent) act like hybrid midfield terriers who have excellent workrate and are proficient at putting in tackles and winning the ball as well.
I am more concerned about our depth in certain areas such as central mid, wing backs and wingers. We have Fletcher, Anderson, Cleverley, Fellaini as cover for central mid whilst we have Young, Valencia as cover for our full backs and wingers as well. So we should clear the deadwood in those areas and replace them with better younger players who can provide excellent cover. We have a good defence(if we play 4 at the back) who have simply found it hard adjusting to the 3-5-2 formation and have compatibility issues as a duo. That should change with Rojo I hope.
People go on about how our players need to learn 352 before we start playing it the right way. Thing is I don't think it's the best formation for United and it's definitely not the best formation for van Gaal who has used 433 far more frequently. He used 352 for Holland out of necessity and it worked well at the World Cup so he thought he could implement it at United with similar outcome only he couldn't - primarly because our players aren't flexible enough tactically to pull it off.
Since our current personnel is better suited to 433, our players are more familiar with that formation (basically 433/4231 is what we have played for a while under Fergie) and van Gaal has had plenty of success with it, why insist on something that may or may not work when 433 is pretty much your certain thing (unless you cock it up by playing wrong players)?