Amid United's push for Brighton midfielder Carlos Baleba, Keane says the squad does need an injection of new midfield talent.
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He said on the Rest is Football podcast: "There's slightly less pressure on their midfield, don't get me wrong, but that will be the problem that'll still hold them back over the next year or two unless they bring in players.
"But they have to start somewhere. They weren't going to bring seven or eight players."
On Baleba, Keane admits he couldn't see the deal being pulled off.
"I'd be surprised if it happens," he said. "I still think he's got to do a bit more. It's still potential. You can't pay £100m for a player that you don't know where it's going to go."