The Spanish midfielder is currently nursing a hamstring injury picked up during a Premier League meeting with Liverpool.
He was one of four players to suffer knocks in that contest, with Ander Herrera, Marcus Rashford and Jesse Lingard also laid low.
Rashford has since returned to action, but Ole Gunnar Solskjaer saw that boost against Southampton countered by Alexis Sanchez damaging knee ligaments.
With the fitness issues continuing to pile up at Old Trafford, the Red Devils are desperate to welcome some of their walking wounded back into the fold.
They are, however, going to have to face key Champions League and Premier League fixtures this week without a World Cup winner, with Mata posting on his personal blog: “I’m writing this today after one of the strangest weeks that I’ve had, on an individual level, in years.
“As I told you a week ago, I’m not used to being injured. The week has crawled by without being able to train with my teammates.
“I’ve been around them each day, of course, as I am going through my recovery at the training ground, but it’s not the same.
“It is hard to not be able to contribute, and even more so during a week with two games. To everyone’s delight, the team have been doing well out on the pitch, showing that we are all able to compete.”
Mata added on the upcoming challenges that he is being forced to sit out: “This week, we have two more games… which will probably go by slowly for me again, although I will be supporting the team and carrying on with my recovery process.
“I’m feeling good about it, every day I’m a bit better and I am working to be back as soon as possible.
“PSG and Arsenal are both top opponents to be facing this week. The game in Paris is particularly tough due to the first leg, but we are United and we never surrender. We have to believe that it is possible.
“Later, we will visit the Emirates again in a vital game as we fight for a Champions League place.”
United trail PSG 2-0 in their Champions League last-16 encounter and have it all to do in a return leg at Parc des Princes on Wednesday.
They are, however, continuing to make impressive progress in domestic competition, with a thrilling 3-2 victory over Southampton on Saturday having lifted them back into the top four.