Rooney has travelled to Munich for United's crucial Champions League quarter-final against Bayern despite fears over a toe injury that ruled him out of United's 4-0 win at Newcastle on Saturday.
The England striker is expected to have a pain-killing injection in the toe on the day of the second leg.
That scenario evokes uncomfortable memories of what happened here at the Allianz Arena four years ago. Rooney injured his ankle in the first leg and was rushed back for the return fixture, only to be taken off after 55 minutes.
The 28-year-old only played three more games that year and went on to have a disappointing World Cup, partly because of the injury.
When asked whether he was risking Rooney's long-term fitness by playing the striker in Germany, Moyes said: "We think it's okay.
"We wouldn't do anything medically wrong. We take advice from our doctors and the people who advise us.
"Everybody knows the character Wayne is and the kind of player he is. He is determined to play and if he is determined we would be mad not to [play him.]"
With the score level at 1-1, United do need Rooney to play in Bavaria. But England need the striker to be match-fit by the time they arrive in Brazil too.
Moyes said last Friday that United would assess Rooney's injury over the weekend to determine the nature of the problem.
The Scot offered up the idea that the injury, caused in the first leg, could be severe bruising, a hairline fracture or a chipped bone.
United recently installed a multi-million pound scanning suite at the Aon Training Complex in Carrington, so it was slightly bizarre to hear from Moyes that United had been unable to diagnose their star player.
"We've not had clarification on the injury yet," the United manager said when first asked.
When pushed on the matter, Moyes conceded that the club did know what the injury was - he was just unwilling to divulge the information with the gathered media in press conference room at the Allianz Arena.
"The doctors know," Moyes said.
What is not in doubt is Rooney's commitment to play in a game in which United must score to make it through to the last four due to Bastian Schweinsteiger's away goal at Old Trafford last Tuesday.
"He is able to train and is massively keen to be involved," Moyes said.
"He's not trained all week so we need to be wary of that. He is happy to take an injection which shows you his feeling for the game and the club and we need him as well. He is a really important player."
Moyes' opposite number Pep Guardiola has been paying no attention to the speculation that Rooney would not start the second leg.
"He will play. I bet you a big glass of beer," the Bayern boss said.
Rooney was all smiles as he trained in front of the cameras at Carrington on Tuesday morning.
The former Everton front man said after the session that he was looking forward to the second leg in Germany.
He wrote on his facebook page: "Good training session with the lads ahead of the Bayern Munich match tomorrow night."
Rooney is also optimistic that United can defy the odds and triumph over Bayern - just as the unfancied Chelsea did in the final two years ago.
The England forward told the official UEFA website: "Of course (I believe we can win it).
"Everyone has their favourites every year - the likes of Barcelona and Bayern Munich - but it does not always happen.
"When Chelsea won it they nearly went out of the competition three times.
"We believe we can go through. We have good enough players.
"It will be tough because in my opinion Bayern and Barcelona are the two best teams left in the competition, so we have to be at our best.
"We are ready. It's an exciting game for us all - the players and the fans - and it's a massive game to be involved in."
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