Defenders Keith Watson and Calum Butcher are both banned following red cards against St Mirren and St Johnstone respectively and will miss the Scottish Premiership fixture at Tannadice.
Boss Jackie McNamara also has concerns over Ryan Gauld (groin) after he missed the 3-0 defeat to the Perth Saints, while Stuart Armstrong and Gary-Mackay-Steven are still nursing the knocks which meant they started the McDiarmid clash on the bench.
Defender John Souttar has issued an ominous warning to their Premiership rivals by claiming the Terrors have yet to hit top form.
The Tannadice men blasted 22 goals during a sensational six-game winning streak that was only halted by a surprise 4-1 drubbing by St Mirren on Boxing Day.
They were then humbled 3-0 by St Johnstone on Sunday as boss Jackie McNamara made eight changes to his fatigued side.
But Souttar has promised that, under the guidance of McNamara, the likes of Gauld, Andrew Robertson and Stuart have still to hit top gear.
He said: "The last two games have been disappointing but I just want to move on and get back to Tannadice where we have played a lot of good stuff this year.
"The gaffer believes in all of his players and I can tell you we have got a lot more to come. I don't think we have been properly at our best in any game this season yet - I mean top form.
"We played well against Motherwell when we beat them 4-0 but I think we can do even better."
Aberdeen boss Derek McInnes watched his side overcome Ross County 1-0, then revealed striker Calvin Zola was an injury concern.
Scott Vernon has been preferred in attack for the Dons in recent weeks, although he missed a penalty in Sunday's game, which was settled by Nicky Low's second-minute free-kick.
McInnes remains hopeful Zola will be fit, but the injury may affect the manager's plans for Wednesday's match.
He said: "Calvin over-stretched his knee in training (on Saturday). He should be fit for United, but that's why he missed out (against Ross County)."
Should Zola not recover, Vernon and Josh Magennis are the other options for the main striker's berth against United, but it is defence that McInnes will be focusing on in January.
He said: "It's got to be the priority now, with Reading's decision on Michael Hector (he will return to the Royals in January when his six-month loan deal expires), Andy Considine tearing his calf and Clark Robertson still some way away. That's not to say that we won't be looking elsewhere, but we are short and we're just about getting away with it just now.
"There's a few options out there, but we'll assess our options. Clubs are reluctant to give out good players in January, and it's a good player we'll need to improve things."