Januzaj came off at half-time during United's 4-0 win over Club Brugge on Wednesday with an injury to his hip area.
After Januzaj came off, Van Gaal pushed Herrera into the number 10 position, just behind main striker Wayne Rooney, and the move turned out to be a masterstroke.
The speed of United's play improved markedly after the switch and the diminutive Spaniard set up Rooney for one of his three goals.
Van Gaal was impressed by Herrera's second-half performance in Bruges and he could be the man the United boss turns to for Sunday's game at the Liberty Stadium.
"I started this season with Herrera as number 10 so it is possible (he will play)," the United manager said.
"I had a theoretical scheme in my head that he could play as a 10, and he did well against Club Brugge.
"Adnan Januzaj is injured so he cannot play
Marouane Fellaini is not suspended any more either so you never know."
Van Gaal missed the Champions League group stage draw - which pitted United against Wolfsburg, PSV Eindhoven, and CSKA Moscow - so he could watch a reserve team friendly against Ajax at Carrington
Tyler Blackett, Antonio Valencia, Andreas Pereira, Jesse Lingard and James Wilson played the whole 90 minutes and Marcos Rojo also featured for an hour.
"It was very important we organised that game," the 64-year-old said.
"It was also against a good team
We won 2-1, that is also good and Sean Goss, who was in the USA, scored the first goal - a fantastic goal - out of a pass from Wilson and Wilson scored the second goal."
United won both legs of their Champions League play-off against Brugge and took maximum points from their opening two league fixtures against Tottenham and Aston Villa.
Van Gaal's team came unstuck against Newcastle last weekend however when they only managed a disappointing 0-0 draw.
Swansea have started the season impressively, taking five points from three matches.
The Swans were the only team to beat United twice last season and Van Gaal is hoping for an improvement on Sunday in his 50th game in charge of the club.
"The first match I played here was against Swansea," Van Gaal said.
"We lost (2-1) and now I have to play the 50th match
I hope we can show how we have improved because that is the main thing.
"Last year we lose only against Swansea all the points and I want always improvement in my team and players."
Van Gaal will have another 48 hours following the Swansea game to make any changes to his squad.
David de Gea, who is yet to play for the club this season, may end up being the main story of transfer deadline day.
The Spaniard is yet to play for the club this season because his focus has been affected by Real Madrid's interest in him, according to Van Gaal.
United are also interested in signing Sadio Mane from Southampton and bids for stars like Thomas Muller, Gareth Bale and even Neymar have been mooted.
Victor Valdes and Jonny Evans have been told they can leave too
Van Gaal was keeping tight-lipped about any potential transfers at his press conference on Friday, however.
"We shall do announcements when we want to do it, not when you want an answer," the Dutchman said.
Van Gaal was a little more forthcoming when asked about United's Champions League draw.
"It could be worse and it could be better," he said.
"We have to analyse the teams
My scout Marcel Bout has already gone to Wolfsburg, they play tonight against Schalke and tomorrow he is at PSV against Feyenoord
We are already busy with a lot of things."
Source : PA
Source: PA