Keane is entering the final season of his contract and, although the Irishman has yet to decide whether to extend his playing career, at 34, he knows his days are limited.
The United manager, Sir Alex Ferguson, has spent the last two years on a fruitless search for a potential replacement, knowing his talismanic captain will leave an enormous gap in his squad when he does finally stand down.
Bayern Munich's Michael Ballack is one possible target, although any move for the new German captain may have to wait 12 months until his contract expires.
In the absence of any alternative, Ferguson has given Smith the chance to show whether he can be a viable replacement, and Smith has revealed that Keane is prepared to offer the former Leeds striker the benefit of his wisdom whenever he wants it.
"Who better could I have to learn off?" he asked. "I have such great respect for Roy and trying to replace him at Manchester United is the hardest thing you can imagine."
Smith caused a mini-rumpus last week when it emerged he had backed out of England's friendly defeat in Denmark to play for United's second string at Bolton. But the England manager, Sven Goran Eriksson, was forced to admit there had been a major communication problem over Smith's absence, with the player completely blameless.
As it turned out, with England suffering so badly in Copenhagen, Smith chose correctly, especially as he scored and got a much-needed 90 minutes under his belt for the reserves.
That extra playing time proved valuable in Budapest on Wednesday night when the 24-year-old occupied the central midfield position in a competitive game for the first time. And although he picked up a booking in the 3-0 win - 6-0 on aggregate - over Debrecen, it did not diminish his enjoyment at being given the chance to fight for a starting spot.
"When you get the opportunity to play for Manchester United, you have to make sure you are ready. That is why I chose to play for the reserves," Smith said.
As a former Champions' League semi-finalist during his Leeds days, Smith knows what it takes to have a long run in Europe's No 1 club competition.
With United having been paired with Villarreal, Benfica and Lille in the group stages of this season's competition, he knows a third successive early exit must be avoided.
"The last 16 is not good enough for a club like this," he said. "We need to win it and we have a good enough squad to do it."
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