On a day when Manchester United fans were encouraged to remember George Best's first appearance for the club, they had two other debuts to celebrate.
Fellaini is an experienced international and United's latest big-money signing.
Januzaj arrived with much less ceremony, signed from Anderlecht as a 16-year-old in 2011.
It's not unlike the story that tells Best's journey from Northern Ireland to Manchester as a scrawny teenager in the early 1960s.
The two Belgians started on the bench against Crystal Palace.
But it wasn't until they were both introduced in the second half that United threatened to grind Palace into the floor.
They were helped by the extra space afforded to them by Kagisho Dikgacoi's dismissal on the stroke of half-time, but that was none of their concern.
Fellaini was on first after replacing Anderson in the 62nd minute.
It wasn't long before he was firing a volley at a sprawling Julian Speroni much to the delight of the fans in the Stretford End, some of whom had doubted the sense in signing the midfielder from Everton on deadline day.
Most of them were converted by the end of Fellaini's first 30 minutes in a red shirt, singing 'United afro, United afro' as he walked off down the tunnel at the end.
But it says much about Januzaj's first Premier League appearance that the fans streaming away down Sir Matt Busby Way at the final whistle were talking about him rather than the new £27.5m signing.
On in the 67th minute for Ashley Young, his comfort on the ball and ambition to attack his full-back belied his 18 years.
There were a few sloppy passes and loose touches, but that's only to be expected when a youngster is making his debut in front of 76,000 fans.
And there was much more to admire than criticise.
Twice he drifted in off the left and glided past two Palace defenders with astonishing ease.
The second time he wriggled all the way into the penalty area but was let down by a wild right-foot shot. There's plenty of time to get that bit right.
Games against the likes of Crystal Palace at Old Trafford is where Januzaj will get his experience this season. He can expect to be involved in the Capital One Cup, too.
It's very early in the youngster's career, but the indicators are that United have uncovered another gem.
After all, there aren't many 18 year olds who can claim to have upstaged a near £30m new signing. And George Best.