The midfielder scored his first goal since 2014 as United won 1-0 at Villa Park to extend their unbeaten Barclays Premier League record at the venue to 20 years.
Januzaj has been linked with a loan move from Old Trafford - with Sunderland reportedly interested - after struggling to make a consistent impact under van Gaal.
The 20-year-old, who was replaced on the hour, failed to score last season but despite netting this time boss van Gaal insisted Januzaj may still go out on loan and called on him to do more.
He said: "You have to wait until we have closed the transfer period
That's my benefit of this period.
"I am pleased with the goal because it was a wonderful goal but you have to do more than scoring a goal and that was less good than the goal.
"I said he shall play 60 minutes which I did with Michael Carrick
It was already decided before the game."
Victory was United's second straight 1-0 win after last week's win over Tottenham and van Gaal believes they can still improve despite their 100 per cent record.
"I'm very pleased because you cannot get more points than six," he said.
"I think we played better than last week, certainly in the first 25 minutes
We kept the ball very well, scored the goal but after we scored I think we were running out of the game.
"We had unnecessary ball losses and then we were not playing what I expect of my team.
"In the second half, especially after the substitutions, I think we controlled the game and could have scored more goals.
"Maybe we should have scored more, because of that there's always tension until the end but we didn't give any chances away."
Memphis Depay wasted United's other best chance when he shot wide with Brad Guzan to beat while Villa struggled to create.
Defeat equalled the longest run of home games without a win against a single opponent in English league history - 20 - but boss Tim Sherwood was not downbeat.
"I didn't think there was too much in the game
The boys gave everything for the cause but we didn't carry enough attacking threat," he said.
"There's a lot of things to improve on
I'm not too disheartened
We'll be okay
I don't take winning on Saturday (against Bournemouth) out of proportion and I don't think losing to Manchester United, with a couple of World Cup winners, is going to destroy my confidence either.
"The attacking force wasn't there tonight, it may be down to personnel, we're missing (Carles) Gil, (Jack) Grealish and Adama (Traore) who might give us a little cutting edge.
"It takes time, we expected that but what they do give me is everything
They try to take instruction on, it's not always perfect but they will improve."
Source : PA
Source: PA