The SPL outfit drew 0-0 with the Italians in the first leg at Ibrox.
The first match at Ibrox finished 3-3 before United triumphed 1-0 in the rematch at Tannadice.
BBC: Cup exit self-inflicted, says Rangers boss Walter Smith
The Ibrox club are now two points behind Celtic, who have played one game more.
Gordon had been left in the stand for last week's Scottish Premier League match at Ibrox.
BBC: SPORT | Football | My Club | Heart of Midlothian | Coach defends leaving out Gordon
The player is an injury doubt ahead of Sunday's Old Firm derby at Ibrox.
The striker was shown a yellow card during Sunday's 3-0 win over third-placed Hibernian at Ibrox.
The 21-year-old was one of the Ibrox club's most consistent performers in his breakthrough season.
BBC: SPORT | Football | My Club | Rangers | Adam commits long-term to Rangers
Adam signed a three-year deal at Ibrox last October after impressing then manager Paul Le Guen.
BBC: SPORT | Football | My Club | Rangers | Adam commits long-term to Rangers
Hearts visit Ibrox on 23 January, with Rangers travelling to Edinburgh again on 27 March.
Three transfer windows have opened and closed without the Ibrox club bringing out their cheque book.
They host St Johnstone at the weekend before testing themselves against Rangers at Ibrox next Wednesday.
It was only Thomson's second goal for the Ibrox side since his arrival from Hibs.
Rangers chief executive Martin Bain said everyone at Ibrox was looking forward to the group games.
The Europa League last-16 tie is finely balanced going into next Thursday's return leg at Ibrox.
Rangers are due to play Old Firm rivals Celtic at Ibrox on 25 March.
BBC: Rangers administrators: 'Old Firm match will be played'
The disturbance happened at Ibrox Subway Station prior to the game on 21 January 2012.
Ferguson received treatment on Tuesday and missed the Scottish Cup quarter-final against Partick at Ibrox.
BBC: SPORT | Football | My Club | Rangers | Ferguson delays ankle operation
Firstly, we have the Gers figures ( Government Expenditure and Revenue for Scotland: not the Ibrox hopefuls).
Ibrox manager Walter Smith has already bolstered his attacking options after signing Kenny Miller and Andrius Velicka.
The former Falkirk, Hearts and Everton man also revealed how much he was enjoying life at Ibrox.
However Weir, capped 69 times for Scotland, hasn't discounted another year at Ibrox or with another club.
Later on, Aberdeen go to Ibrox looking for their first success on Rangers' turf for 17 years.
Jeremain Lens tapped the Dutch side in front on 15 minutes as the visitors dominated possession at Ibrox.
Rangers ended the season by conceding two stoppage-time goals as Motherwell fought back to draw 3-3 at Ibrox.
The Ibrox outfit have been drawn to face Sevilla, Stuttgart and Romanian champions Unirea Urziceni in Group G.
The Ibrox club will have to play four games in eight days in the chase for the title.
And the diminutive Ibrox striker had the chance to put the game beyond Celtic with five minutes remaining.
The defender has been troubled by injury during his time at Ibrox and made just 16 appearances last season.
The Ibrox side were spraying the ball about with confidence after the break as Celtic struggled to get possession.
Following heavy defeats to Sevilla and Unirea Urziceni at Ibrox, Rangers' deficiencies at this level were again cruelly exposed.
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