Juninho has not been given "a fair crack of the whip" by the Celtic manager, Martin O'Neill, the player's agent complained yesterday.
The Brazil international arrived in Glasgow earlier in the season and made an immediate impression in the Old Firm derby against Rangers in August. But the 31-year-old has found himself on the sidelines following a poor display in the Champions' League against Shakhtar Donetsk in Ukraine last October.
The former Middlesbrough player has been linked with a January transfer window move away from Parkhead. But, although he is becoming increasingly frustrated by his situation, his agent, Brian Hassall, said his client is prepared to fight for a first-team place.
"He is terribly frustrated," Hassall said. "If O'Neill had 15 players of equal ability in the squad, then I could understand it. But how many of those players have got over 50 caps for Brazil? We are not talking about Outer Mongolia here, we are talking about 50 caps for the best country in the world.
"That doesn't mean that he will not be fighting for his place. All he is asking for is a fair crack of the whip. If somebody does not play particularly well, then you expect to get your place back in the team.
"He held his hands up when he didn't do well against Shakhtar Donetsk in the Champions' League, but he wasn't the only player who played badly. Others have carried on, but he is just not getting a look in.
"He is a lovely lad and loves the fans up there. He has just moved into a beautiful house and you don't buy a beautiful house if your intentions are to leave. He is not a quitter and will fight for his place, but the future depends on whether the manager wants him."
However, Hassall has played down a potential link with West Bromwich Albion and Juninho's former manager at Middlesbrough, Bryan Robson, who twice took him to Teesside - once on loan.
But Hassall said: "Nobody has said anything to me. West Brom are rock bottom and I couldn't imagine he would want to go there, as much as he likes Bryan. When you think about it, Bryan has also got Kanu there and they are too similar. I am not saying Juninho is not better, in my opinion, but I would be quite surprised if they wanted him."
Back in Brazil, Romario, Juninho's former international team-mate, announced his retirement yesterday. The 38-year-old, whose final club was Fluminense, said: "I've stopped. I can't do it any more. I don't have the desire."