#1 Manchester United's dismal form in recent weeks
Since their win against PSG, the team has lost three out of four games they played. They lost the games in an unimpressive manner. Getting knocked out by the Wolves in a shocker at FA Cup and losing to them in the subsequent Premier League clash doesn't look positive.
They lost against Arsenal in one of those games without even getting a goal. Their only victory in three games came against Watford. However, the victory against the underdogs came with a result of 2-1.
Manchester United are by no means a weak side. They have a good bounce-back ability as witnessed in the recent clash against PSG. However, they have to answer a much stronger and dominant Barcelona.
If they had no answer for a comparatively fragile Wolves and a much stronger Arsenal, then it seems like they may not have it against the La Liga table-toppers. This is a reason why they need to be prepared for the worse.