Actually there are another two massive differences (though not as big as De Gea). AWB and Shaw instead of Valencia/Young and Rojo/Young.I think 15/16 is much more fair comparison than 16/17 & 17/18. Team under Mourinho will always look good defensively, the guy can convince some people to believe Eric Dier will be a top centre back.
Either way. There is one massive difference here. DDG was on his prime saving everything back then.