The former Arsenal captain was expected to take training as normal at the club’s Beckenham base before he was required to leave due to an urgent issue.
Roberts filled the void by leading Palace’s penultimate session before the weekend fixture and then took on press conference duties.
“Patrick was in as normal this morning preparing for training. Unfortunately, he received a phone call and had to leave immediately to attend to an urgent family matter,” he said.
“My thoughts are first of all with Patrick and the family but I’m just here trying to make sure collectively as a staff we rally round.
“We have a good group of staff and the players as usual have been professional in the way they applied themselves so we had a good training session.”
Ex-Wales assistant Roberts admitted the absence of the Eagles boss does make a difference but he was hopeful Vieira would return this weekend.
Asked if the Palace manager would be able to attend the match at United, the number two added: “I hope so. We obviously just finished training, just come off pitch so I haven’t spoken to him since training, but I will and I would expect normal service would resume tomorrow.
“It makes a big difference when Patrick is not here because he is our leader, the manager, and you always want him at the helm leading matters.
“Of course we have a responsibility in this unfortunate circumstance to do everything correctly to make sure everything goes as seamless as possible, as smooth as possible until he is back with us.”
If Vieira does step out at Old Trafford this weekend, he will face the third manager to sit in the United dugout this season.
Ralf Rangnick was granted a visa on Thursday to officially begin his tenure as interim boss, which is due to last until the end of the campaign.
Michael Carrick left United following their win over Arsenal in midweek after he had stepped up to be caretaker when Ole Gunnar Solskjaer was sacked last month.
Roberts said: “Of course when a new manager comes in there is obviously the potential for some changes even in the short period that he would have been with the team.
“We will have to wait and see but really we will just focus on what we normally do. We will focus on our own performance and try to give one that will get us something out of the game.”
Joel Ward will be missing for Palace due to a one-match suspension for receiving his fifth yellow card of the term in Tuesday’s defeat to Leeds.
Nathan Ferguson (fitness) and James McArthur (hamstring) are also expected to be absent while Joachim Andersen, who also has a hamstring issue, will need to be assessed.