United made an opening €30 million (£25.9m) offer to Barcelona in their attempts to sign the former Arsenal captain on Monday but the ex-Red Devil argues that the Everton star would bring more of a goal threat to David Moyes's midfield.
"What Fellaini gives you, which Fabregas doesn't, is goals," Macari told talkSPORT.
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"They desperately need someone in there. There are one or two players missing at the moment. Ashley Young is out injured, Antonio Valencia is not on the Far East tour, Ryan Giggs is not getting any younger and Paul Scholes is gone.
"People were screaming last season for a midfield player to come in and this year the need is even greater."
Another ex-Manchester United stalwart, defender Denis Irwin, agrees with Macari's comments that United need to strengthen in midfield.
"With Scholesy retiring, you are losing a fantastic footballer and, with Darren Fletcher [who suffers from a chronic bowel condition], we're not quite sure when he's going to come back," Irwin remarked.
"So it might need strengthening but I am sure David Moyes will have his own ideas."