Diouf has made only five appearances for the Yorkshire club this season, which has failed to get going for the Senegal international after he returned from Africa with a shin infection in the summer and struggled to win back a place in McDermott’s side.
The 32-year-old has since spent time back in Senegal for personal reasons. He was deeply affected by the death of former national coach Bruno Metsu, while the two-time African player of the year also attended Nelson Mandela’s funeral this week.
McDermott confirmed Diouf will not be included in the squad for this weekend’s derby clash at Doncaster.
“That won’t happen,” McDermott said. “It’s a difficult one for Dioufy. He’s spent quite a lot of time away, travelling over to Senegal and has had a few things he needs to sort out personally as well.
“It’s been difficult for me to be able to give him an opportunity because quite frankly he’s not been around the training ground as much and it’s been a tough one for me.
“That’s where we’re at with him at the moment.
“I’m not saying there’s an issue. There isn’t. But training-wise with the group – he’s not done a lot and to give himself a chance he has to be training and be right up to speed.
“The circumstances are what they are, but, from a football point of view, you have to be in and around the training ground, you have to be up to speed,” said McDermott.
“He’s got a few personal things that are happening as well and sometimes life is what it is and there are more important things than football that you have to take care of.”
United have signed former Southampton trainee winger Gboly Ariyibi until the end of the season.
The 18-year-old received a short-term deal after an appearance for United’s Under-21s at Thorp Arch last week and he is already training with Brian McDermott’s senior squad.
Ariyibi was raised in the USA and joined Southampton’s academy as a 10-year-old in 2005.
McDermott said: “He’s only 18 and we kind of knew him already.
“He played in a couple of games and took part in some training sessions and he looks like he’s got a chance.
“He’s quite an exciting player – very, very quick. He’s a great lad too.
“He really wanted to sign for us and he’s been training with the first team. He looks good.”
Birmingham defender Mitch Hancox has been ruled out until the new year with a knee injury.
The left-back, 20, was forced off late on in last weekend’s 2-2 draw to Middlesbrough and scans have confirmed a ligament strain.
Manager Lee Clark said: “Mitch has taken quite a serious smash to the knee, as you can imagine with the ferocity of the tackle, so he won’t be with us until the new year.”
Crewe have confirmed they have activated a one-year extension in striker Mathias Pogba’s contract.