Tony Mowbray’s men conceded two goals in the final seven minutes to give the Arabs their first victory over the Glasgow giants since August 1999.
United striker Jon Daly cancelled out Barry Robson’s penalty opener for the Celts before Darren Dods powered home a header in the final minute of the match.
The result leaves Tony Mowbray’s side a point behind Rangers who cemented their place the top of the league with a 3-0 win over Kilmarnock.
Kris Boyd opened the scoring for the Gers against his former club with his eighth goal in nine SPL starts. The striker has now scored 149 times in the top flight, nine goals short of Henrik Larsson’s record.
Kenny Miller and Steven Whittaker were also on target for the Gers who face VFB Stuttgart in a must-win Champions League tie midweek.
The Ibrox side were without Algerian defender Madjid Bougherra who failed to return from international duty on time after his side’s dramatic play-off win over Egypt.
Aberdeen midfielder Gary McDonald was shown a red card for pulling his shirt over his head and patting his stomach in tribute to his pregnant wife after equalising for the Dons against Motherwell.
Lukas Jutkiewicz had given Well the lead after 13 minutes before McDonald headed home a Charlie Mulgrew cross at Fir Park.
After the match Dons boss Mark McGhee slammed referee Crawford Allan for showing his midfielder a second yellow for his celebration.
He said: Ã¢â‚¬Å“The sending off is ludicrous. Getting sent off for something like that makes a mockery of the game.Ã¢â‚¬Â
Bottom of the league Falkirk grabbed a welcome three points with a 2-0 home victory over Hamilton. Thirty-eight-year-old Lee Bullen scored his side’s second after Chris Mitchell had put Eddie May’s side ahead.
Hibernian’s dream of splitting the Old Firm was dealt a blow after they were held 1-1 at St Mirren, while Derek McInnes’ St Johnstone came from behind to beat ten-man Hearts 2-1 at Tynecastle.
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