Januzaj has been sidelined recently through illness whilst Herrera has had an ankle problem.
Herrera had recently been struggling with an ankle problem, whilst Januzaj was ill, and the United boss has said he will look to use his squad a lot more in the coming weeks.
He told reporters: "Herrera trained today for the first time with us but, yes, they don’t have match rhythm. Januzaj has also trained two training sessions with us, but he was ill and you have to recover from your illness, not from your tiredness. Maybe it’s worse to recover from illness.
"There are players coming back, we have seen the first signs of that in the last two games with Rafael, Chris Smalling and Luke Shaw. They don't have any match rhythm and that's my problem, it's the case with a few players. We played three or four games in the space of a week, so we need these players now more than ever. It's now difficult for me to select the team."
No further players are due to return to the side according to van Gaal, meaning Angel Di Maria and Marcos Rojo are among those who will miss the trip to the Britannia Stadium.
At the halfway stage of the season United sit third in the Premier League. Following a difficult start to the campaign, United have won seven of their last nine games - three points clear of Southampton and Arsenal in fourth and fifth place respectively, and van Gaal's desire for continued improvement remains, and had kind words for the support the fans have given the team so far this term.
He added: "I think everyone can see that we are improving every week in spite of a lot of injuries, and that’s because of the philosophy. I think the players understand the philosophy, and that’s why we are improving – but we can improve more when we have all the players available.