Rangers announced the United States captain's signing from St Etienne on Wednesday and he played in a 2-1 first-leg defeat in Slovenia the following day after receiving international clearance.

Bocanegra was forced to sit out a 3-0 win over Motherwell on Sunday because he had not been granted a visa to work in the UK. That sparked a complaint from Maribor, which UEFA confirmed their control and disciplinary body would investigate.

Rangers are confident they will be cleared over the issue, but it is unclear whether Bocanegra will be able to feature in the second leg at Ibrox tomorrow night.

In a statement on their official website, the club said: "We are extremely surprised by the complaint of NK Maribor.

"As confirmed by the SFA, Carlos Bocanegra was deemed eligible to play at a domestic level within the UEFA deadline in accordance with the requirements of the UEFA Regulations.

"We have at all times acted with due regard to the requirements of the UEFA Europa League Regulations and we are confident that the complaint of NK Maribor will be rejected by UEFA."

The development is the latest selection headache to hit Rangers manager Ally McCoist, although he is hopeful his lengthy injury list will shorten before the Maribor clash.

Kirk Broadfoot, Sasa Papac, Steven Whittaker, David Weir, Maurice Edu, Lee McCulloch, Kyle Bartley, James Beattie, Jamie Ness and Kane Hemmings were all unavailable through injury or suspension against Motherwell while new signings Alejandro Bedoya, Matt McKay and Bocanegra were awaiting visas.

Edu will return from suspension, although Steven Naismith is ruled out after picking up his second booking in Europe this season in the first leg in Slovenia.

Whittaker, who has been struggling with a broken toe, is also out after losing his appeal against a three-match ban from UEFA for his red card in the Champions League qualifier in Malmo.

Bocanegra has received his visa but McKay and Bedoya are still unavailable, however, there may be some light at the end of the tunnel for McCoist.

"We are hopeful that Broadfoot, McCulloch and Ness, boys like that, could be back," McCoist told Blues News.

"We will give them as much as time as they need but I would certainly like to see our ward at Murray Park getting emptied.

"For our game against Motherwell we had eight or nine players injured which is a big slice of bad luck.

"But if some of them are available to us for Thursday night or Sunday we will be very pleased.

"There is probably a bigger doubt about Kyle (Bartley) being back than the others I have just mentioned.

"I am not ruling him out altogether but he still has a little bit of a problem in the groin area."

There was bad news over young English striker Kane Hemmings, who made his competitive debut for Rangers in the Champions League qualifier in Malmo, has been ruled out for between two and four months.

The 20-year-old has been told he needs surgery on his knee after missing spells during the past two seasons with similar problems.

And McKay is unlikely to be involved in the Clydesdale Bank Premier League visit of Aberdeen on Sunday.

The 28-year-old midfielder expects his visa application to be granted later this week but McCoist feels it is unwise to make him travel over from Australia, when he is due to return for World Cup qualifiers against Thailand on September 2 and Saudi Arabia four days later.

However, Bedoya, who is ineligible for the Maribor clash, will be in contention to face the Dons after receiving his visa.

McCoist said: "It's great that Alejandro will be with us this week but Matt will be different.

"I'm not sure it would be a good idea for him to fly across the world then back again in the space of three or four days."