Lukaku, Linguard, Martial, Rashford, Young - to name a few that I think Sanchez' pace and strength will match up better with.
And, as you say yourself, we have no way of knowing just how fit/unfit Alderweireld is. We do know, however, players rarely are up to speed after one game in which they basically walked through. And it looked to me like he showed a lack of confidence in his pace by positioning very deep, against a side hardly blessed with pace.
Its common sense to assume he isnt 400%, and in a cup semi final against a top side - I think its common sense to err on the side of caution.
Hence Id play him in a back 3, keep Sanchez pace on the field. But I wouldnt play him if its a back 4, as Id prefer a fully fit (and rested) Sanchez.