The 71-year-old was present at Old Trafford on Sunday as United drew 2-2 with Tottenham.
Ratcliffe has purchased a 25% stake in the club in a deal worth £1.3bn, though the Premier League must first ratify the purchase.
The deal will see Ratcliffe take full control of football operations and will enable him to appoint a sporting director and head of recruitment.
Both will report to Sir Dave Brailsford and Jean-Claude Blanc, who are expected to take seats on the club’s board.
But Keane says Ratcliffe faces major challenges, and needs ‘time’ to fix the amount United have been paying in transfers, on salaries and on what players.
‘How long have you got?! No, it’s going to take time,’ Keane told Sky Sports.
‘We’re hoping there’ll be better decision-making at the top of the club, particularly in regards to recruitment which hasn’t been good enough, the fees they’ve been paying, the contracts they’ve been handing out.
‘But he’ll need some time, he’s got good people around him and he’ll need to get a feel for everything before he can start to take Man United back to the very top.’
Ratcliffe hopes to have his deal ratified in early February.
United will then look to appoint a CEO, with Blanc a leading contender.
Dan Ashworth is the favourite to take charge as sporting director, though Newcastle United are hopeful he will reject the move.
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