Teenage winger Januzaj made only his fifth Premier League start of the season in Saturday's 3-1 victory over relegation-threatened Leicester City.
The Belgium international caught the eye against Nigel Pearson's side, but Van Gaal warned the 19-year-old that he has to produce the goods on a regular basis in order to play a bigger part at Old Trafford.
He said: "Every time you have to compare and that's also the same with [Darren] Fletcher, who is a very good player, but I have to compare him with Michael Carrick and Daley Blind who are playing in that position.
"The competition is high and also that's difficult to manage for players. Also for the player himself it's difficult to manage.
"That’s why I want to help Fletcher [to secure a move away from Old Trafford].
"It is also like Januzaj – I have to compare him with [Juan] Mata, but in the next press conference you shall ask, 'Why? Why? Why is Mata not playing?'
"I have to choose and now I choose Januzaj, but I’m very happy you ask about Januzaj and not for Mata.
"I'm looking every week and every day the players have to show it. I've said that to players, I've said to [Radamel] Falcao, I've said to Mata and I've said to Januzaj. But I'm very pleased that you [the media] are pleased with his performance."
Van Gaal's side moved up to third in the table at the weekend and they will be expected to progress past League Two Cambridge.
Richard Money's men deservedly secured a money-spinning replay by holding the illustrious Premier League side to a goalless draw at the Abbey Stadium.
Cambridge had their chances to claim a shock victory in that stalemate, but it would take a Herculean effort to see off Van Gaal's men at Old Trafford.
The visitors make the journey north on the back of a 3-2 defeat at Luton Town on Saturday which left them 14th in the table.
Johnny Hunt and Robbie Simpson scored after coming off the bench at Kenilworth Road to give Money something to think about when he comes to naming his side on Tuesday.
The fourth-tier outfit will be backed by over 6,600 supporters at Old Trafford and that huge contigent will make the trip with hope rather than expectation of pulling off one of the biggest FA Cup shocks in the history of the famous competition.
The likes of Marouane Fellaini, Ander Herrera and Mata will be hoping for a recall for the hosts after being named among the substitutes against Leicester.
The winners of Tuesday's tie will face a trip to either Preston North End or Sheffield United in the fifth round.
