Consistency is key behind any solid defense which in turn is key to success. Think about AC Milan star studded defense (Tassotti-Costacurta-Baresi-Maldini), United's treble defense (Gaz-Stam-Johnsen-Irwin) or the third CL winner (Wes-Rio-Vidic-Evra). We knew what the defense lineup was. That's a problem with Shaw. He was always an injury prone player and I can see things getting worse the older he gets. On top of that, Shaw is on 150k a week. We can't keep a squad player on that salary especially since he is hardly reliable (due to injuries not talent). Thus a first teamer is indeed needed.
Malacia is a squad player at best and that assuming he returns from injury the player he once was. Amass on the other hand needs a year of regular football something United can't provide.
The risk of a repeat of next season is real and something that United can't afford.
In my opinion we should buy a first team LB and loan Amass till January. I understand that the budget is tight but that's exactly were scouts/recruitment team need to come in and shine. If by some miracle Malacia and Shaw remain fit then we can extend Amass loan which in turn should provide us with a good squad player for next season. Irrespective of Shaw's form by the end of the season he simply need to be phased out. Someone on that salary needs to be available on a regular basis something a then 30 year old Shaw can't provide.
Getting a LB is as important as getting a top CB and CM, imo. I actually hoped that we would upgrade when Shaw's contract had an year left. However, in true United fashion we gave him a bumper long term contract, half of which he has spent on IR. Malacia is simply not good player and hopefully he would be moved on as soon as he gets fit. Shaw should be used as a back up for both LCB and LB. In fact, he should think about moving to LCB permanently if wants to prolong his career.