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 minutes and got 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
You don't believe that, I think!
"I look at players every day
I observe and I communicate with my players what I want
And the moment I think, 'OK, he is now ready to play,' then maybe I can let them play."
- Pressure 'all on United' -
The FA Cup is the only competition that United stand a realistic chance of winning and with Manchester City, Chelsea and Tottenham Hotspur among the teams to have fallen by the wayside, the field has opened up.
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.
Perhaps distracted by the prospect of the trip to Old Trafford, Cambridge's league form has not matched their efforts in the cup, with Richard Money's side collecting just one point from their last two League Two games.
Saturday's 3-2 defeat at Luton Town, which left Cambridge in the bottom half of the English fourth tier, provided a wake-up call.
The replay will give an opportunity for midfielder Luke Chadwick to return to the club where he started his career, while winger Ryan Donaldson also has some experience of the stadium.
Aged 17, Donaldson sat on the bench at Old Trafford for his former club Newcastle United.
"Kevin Keegan was the manager
I never got on, but it was a fantastic atmosphere," he recalled
"And the pitch seemed huge
Maybe that will suit me if I am in a wide role again.
"But no matter what happens, it will be an incredible night
And the pressure is all on them, isn't it?"
The winners of the game will visit either Preston North End or giant-killers Sheffield United, who fell to Tottenham in the League Cup semi-finals last week
The two third-tier sides meet at Bramall Lane.
In Tuesday's other replay, Premier League Sunderland visit Fulham, while Liverpool travel to second-division Bolton Wanderers on Wednesday with a fifth-round trip to Crystal Palace up for grabs.
Fixtures (1945 GMT)
Tuesday:
Fulham v Sunderland, Manchester United v Cambridge United, Sheffield United v Preston North End
Wednesday:
Bolton Wanderers v Liverpool
Source : AFP
Source: AFP