Giggs backs duo to shine in Brazil
Rooney will miss a third successive game for Manchester United on Sunday due to a groin problem, although Giggs is sure the England forward will be fit enough to go to the World Cup.
But while Rooney is a household name opposition teams will be well aware of, Januzaj is something of an unknown threat.
The 19-year-old may have had a good season with United, but he is yet to earn an international cap. The midfielder only pledged his allegiance to Belgium - the country of his birth - last month.
David Moyes used Januzaj sparingly towards the end of his tenure as United boss as he feared the teenager would break down due to exhaustion.
But Giggs has no qualms about Januzaj heading to Brazil with his new international team-mates this summer.
"Adnan is the kind of player who could light up a World Cup, whether that is starting games or off the bench," said the United interim player-manager, who started Januzaj in the 3-1 win over Hull on Tuesday.
"I was excited when he got the ball the other night - when I was on the pitch and on the bench.
"He floats with the ball effortlessly. He is just one of those players you love watching."
Belgium are outsiders in the running for World Cup glory, but with the likes of Eden Hazard, Romelu Lukaku, Januzaj and Simon Mignolet among their ranks, they have the potential to reach at least the quarter-finals.
England will hope to reach that stage in the tournament too. And they will rely heavily on Rooney to get there.
Rooney is England's most dangerous player. His power, pace and eye for goal will worry Italy, Costa Rica and Uruguay, who make up Group D.
But the fact remains that the 28-year-old has yet to live up to his billing on the global stage.
Rooney's previous two World Cup campaigns have been hampered by injury, and his best international tournament came 10 years ago, when he burst onto the scene at Euro 2004.
Giggs is sure Rooney can be a leading star for England this summer, though.
"Wayne is at the peak of his powers," said Giggs, who has played alongside Rooney for 10 years.
"Wayne is a top player. He is at a good age.
"I know for a fact he can't wait for it.
"He has had a fantastic season and he is ready to make an impact."
Rooney broke a metatarsal in April 2006, limiting his impact at the World Cup in Germany, and four years later he suffered an ankle injury just before the tournament in South Africa.
The former Everton striker recently had a spell out with a toe injury and he now has a groin complaint.
The injury will not stop the striker scoring goals at the World Cup, according to Giggs, who is also confident that Phil Jones will be named in Roy Hodgson's 23 on Monday despite a shoulder injury.
"Wayne will be fine for the World Cup," Giggs said.
"It was just a little groin strain. He is fine. He has done all the work in the gym.
"He trained yesterday. He will be fine. He is going away next week and working on his fitness as well so it has probably helped him.
"The injury has probably done him a favour."
Sunday's trip to Southampton will be Giggs' final game in charge of United.
When asked whether his short reign had given him more of an appetite for a career in management, Giggs said: "Yes.
"Sometimes you probably think I could do without it and other times it gives you so much pleasure.
"It has definitely helped me prepare for what will come after my playing career."
Giggs remains undecided on what he will do next year.
He would like to be Manchester United manager one day, but he does not hold it against the board for looking to appoint a more experienced candidate this summer, like Louis van Gaal, who could be appointed next week.
"There is no major disappointment," Giggs said.
"I have plenty of time (to manage)."
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