The pair joined from Bayern Munich for a combined fee of &poubd;60m.
On De Ligt, Ashworth said: “Matthijs is an outstanding defender and proven winner who will add quality, experience and leadership to our existing group of senior players.
“He has already enjoyed considerable success in his career, and we are convinced that his best years are still ahead; we want to help him achieve them at Manchester United.”
And on Mazraoui he also stated: “Noussair is a dynamic, attacking full-back who can play on both the right and left side of defence. His experience and versatility will make him an excellent addition to our squad.
“Having already won a number of trophies in his career, we know that Noussair’s winning mentality will be a real asset to the group as we work together to deliver the success required of Manchester United.”