The 19-year-old Belgium international was one of the few success stories during the disastrous reign of Van Gaal's predecessor, David Moyes.
At the corresponding stage of last season, he had made 24 appearances and scored three goals, including two in a 2-1 victory at Sunderland that heralded his arrival on the Premier League scene.
But under Van Gaal, Januzaj has been involved in only 14 games to date this season, with his start in Saturday's 3-1 victory over Leicester City just his fourth of the campaign and first since November.
"I've not been playing for so long so it was good to get 90 minutes, even if I was really tired in the last 15 and got a cramp," said Januzaj.
"Of course we want to win the FA Cup and this is a big chance to win it. We were good going forward against Leicester and could have scored more than three goals, but hopefully we can take this into Tuesday night and win the game."
Van Gaal was more expansive when discussing the future of Januzaj, who turns 20 this week.
It was predicted that the youngster would be allowed to leave the club on loan during the January transfer window, but the United manager has been impressed by Januzaj's efforts of late and chose instead to keep him at Old Trafford.
"He has a lot of talent, but you have to perform that talent in the matches. And also in the training sessions," Van Gaal told reporters.
"I'm not making the line-up. The players themselves are making the line-up.”
According to the United manager, he observes players on a daily basis and then decides whether to let them play or not.
United, embarrassed by a 0-0 draw in the first meeting between the sides, therefore seem certain to field a strong line-up, with Van Gaal claiming he will resist the temptation to hand a debut to new goalkeeper Victor Valdes.
In Tuesday's other replay, Premier League Sunderland visit Fulham.
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