The signing deadline passed without Celtic announcing the completion of a proposed loan deal for Ghana winger Wakaso Mubarak from Rubin Kazan.
Celtic had agreed terms with the Russian top-flight club over a one-year deal for the 24-year-old.
But Celtic manager Ronny Deila had not been expecting Wakaso to play in Wednesday's Champions League tie against Maribor.
"It is going on, but it is hard to get the paperwork together," he said.
"I am not 100% sure, but I think it is going to be hard to have him ready for the game with Maribor."
Wakaso, who was undergoing a medical in Moscow, has 19 caps for his country and appeared as a substitute in Ghana's 2-2 draw with Germany during this summer's World Cup finals in Brazil.
He has been with Rubin Kazan in Russia's top flight since summer 2013, having previously played for Espanyol in Spain.
Wakaso began his career with Ashanti Gold in his homeland before moving to Spain, first with Elche then Villarreal before switching to Espanyol in 2012.
Prior to the trip to Slovenia to face Maribor on Wednesday, Celtic assistant manager John Collins said: "He's got pace in the wider areas and a good delivery with his left foot, so that's what we're looking for - to add pace at the top of the pitch."
The Scottish champions had also been interested in Stoke City striker Cameron Jerome.
However, the Glasgow club are not optimistic of securing the 28-year-old as they believe the Englishman would prefer to remain in his homeland.
Jerome, who began his senior career with Middlesbrough before spells with Cardiff City and Birmingham City, spent last season on loan to Stoke's Premier League rivals, Crystal Palace.