The Tannadice player limped off during last Wednesday's 0-0 Clydesdale Bank Premier League draw with Inverness after he was challenged by Caley Thistle defender Josh Meekings.
The tackle occurred just 16 minutes into the game at the Caledonian Stadium but Russell played on for a further 22 minutes before eventually being replaced.
Now a scan has shown the player - who has scored 20 goals in 35 outings this season - has suffered a broken fibula.
The player broke the news when he posted on Twitter: "Absolutely gutted got an x-ray this morning on my injury and I have fractured my fibula."
Russell missed United's 2-1 derby win over city rivals Dundee on Sunday as the Tangerines booked their place in the William Hill Scottish Cup semi-finals.
They will now have to take on Celtic at Hampden on either April 13 or 14 without the Scotland Under-21 cap.