Celtic have been told they must secure an official agreement with Toronto FC and Major League Soccer for Dwayne De Rosario to continue his trial period.
De Rosario trained with Celtic at Lennoxtown on Tuesday and manager Neil Lennon declared the Canada midfielder would be there for a week.
But both his club and the North American league later insisted they had been unaware of the trial and had not authorised the player to train with Celtic.
Toronto had dismissed reports on Monday that the 32-year-old was going on trial only to see photographic evidence to the contrary from Celtic`s training ground the following day.
The official MLS website later reported that the league had been in contact with Celtic in a bid to sort out the misunderstanding.
"We talked with Celtic chief executive Peter Lawwell and he indicated that Celtic had been told by both the player and his agent that they
had permission," a league official told www.mlssoccer.com.
"When we are told that by a club like Celtic we are inclined to not move towards a legal action and instead work with them to secure the proper paperwork."
Toronto are keen to ensure they are not liable should injury strike their contracted player, who has scored 15 goals in 55 internationals.
A club statement read: "We were unaware of Dwayne's plan to train with Celtic and we will contact them to understand the situation.
"Obviously, if there is interest in a short-term loan arrangement by
either him or Celtic, they will need to approach us and the league formally, and we will consider based on what is in TFC's best interest."
Toronto interim director Earl Cochrane added: "I was contacted a little more than a month ago by Dwayne and his representative asking if we would entertain a loan or training stint.
"We said we would but would need assurances and final sign-off on it - that an agreement would need to be in place that we were comfortable with."
"They understood that completely. I have not received anything official from Dwayne or his agent or Celtic.
"MLS will reach out to Celtic to clear up this situation and will do what is best for Toronto FC."
Lennon had clearly been unaware of any potential problems when he spoke about De Rosario on Tuesday morning.
Lennon, who has also taken Freddie Ljungberg on trial, said: "He is in today. He's a similar player to Shaun Maloney, he likes to play off the left and come inside. He is a skilful, sharp and intelligent player.
"Again it's an opportunity for us to have a look and see what he can offer us, and he's here for a week."