The Tottenham team he played in was not better than your current squad, although I do accept that the overall level in the Premier League of all clubs has increased since the days Berba played for Spurs so we'll agree to disagree again
I don't know how better to explain the "who would you drop" question as I've tried a number of times but will give it one last go:
Irrespective of formation, every team needs 1) a goalkeeper 2) at least 2 centre backs 3) either two full backs or two wing backs depending on system 4) at least one Centre Midfielder 5) at least one central striker. That's seven players, meaning you can fit the other 4 outfield players into whatever formation you choose, but pretty much every single formation employed by any top team has at least the above 7 positions.
Before we go any further we'll deal with the full backs or wing backs question. Shaw is your best left back and Wan-Bissaka is your best right back. Both are better at defending than attacking, AND if you play wing backs you need a 3rd DC or a fantastic ball winning midfielder to sit just in front (Owen Hargreaves is the last one you had albeit injuries finished him which was a shame).
So we've decided on a back four with traditional full backs giving us GK - De Gea. Two best DC's are Mcguire & Lindelof & Shaw at left back, Wan-Bissaka at right back.
Popular formations with a back four - 442, 433, 451, 4231.
442 - Traditional Man U favourite employed by Fergie many times over the years with some wonderful partnerships up front. Won the treble with this formation but I don't think you have the quality strikers or the wingers to beat a man and put crosses in (maybe with the exception of James who I really like).
433 - Think Liverpool. The "wingers" are effectively inside forwards who are in the team to get into the box and score goals, not to take their full back on and get crosses in from the byline like a traditional winger.
451 - Defensive formation, packing the midfield to contain superior opposition and try to grab a goal on the counter. Not really the Man United ethos.
4231 - More attacking version of the 451 with two holding central midfielders (can be deep playmakers so not just ball winners), two advanced wide men (can be inside forwards or wingers), a CAM just behind the striker working as the focal point to pull the strings and chip in with goals and one up top who has the responsibility of not only scoring goals but also holding up the ball whilst teammates catch up with play.
Given your current squad, I'd try either the 4231 or 433.
4231 - Drop Martial for Berbabov. Rashford playing inside forward, James as a traditional winger. McTominay on defensive duty, Pogba more roaming box to box.
433 - drop James for Berbatov. Both Martial & Rashford supporting strikers, problematic for Martial as he's right footed.
Essentially you need a top striker and, unless we are saying James is good enough, you need a top right winger / left footed inside forward to play on the right if you were to adopt either of the above two formations.
Williams / Dalot interchangeable for Shaw / Wan-Bissaka
Fred / Matic could come in for McTominay or Pogba
Greenwood backup for Berba
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