Dedryck Boyata has not played for Celtic since returning from World Cup duty
Dedryck Boyata let down his Celtic team-mates, the club's fans, and himself by not playing against AEK Athens, says Paul Lambert.
Manager Brendan Rodgers said the Belgium was fit to play in Tuesday's Champions League qualifying defeat.
But defender Boyata, who was a target for Fulham last week, insists he is "not yet 100% operational".
"If he's fit, he has let himself down," former Parkhead midfielder Lambert said. "I think he's made his bed."
Boyata failed to travel to Greece with the squad for the second leg of the third qualifying round tie, and has yet to make an appearance this season.
The 27-year-old, who was with Belgium at the World Cup earlier in the summer, posted a video of himself on Instagram receiving treatment before the game.
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Lambert says Boyata, who joined Celtic from Manchester City in 2015, has been "badly advised" and that the situation was "a distraction" for Celtic.
"It didn't help them," he told BBC Scotland after two defensive lapses allowed AEK to win 2-1 on the night and 3-2 on aggregate to progress to the play-off round.
"Maybe the stronger ones in the team will go back and say, 'OK, if you don't want to play with us, don't. Don't try to come back in'.
"I don't know how he can win the Celtic supporters round. I don't think they will be too forgiving."
There have been reports of unrest between Rodgers and the board, with the manager saying after the defeat that it is "not rocket science" to say they should have made more signings in the summer.
Lambert agrees that the Scottish champions have paid the price of not being active enough in the transfer market, but does not think any rift is unrepairable.
"Looking at Celtic at the minute, I'm not sure they've got a big enough squad to go and compete on all fronts," he said.
"I don't think Brendan said anything derogatory to the board. I think he's just stating what every manager wants to do, strengthen their team.
"And I don't think Brendan is the kind of guy who would walk away - he is not that type."
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Paul Brennan, blogger for Celtic Quick News, on BBC Radio Scotland
The Boyata situation really stretches back to January. Celtic brought Marvin Compper in to allow him to be sold this month - I'm pretty sure that was the club's plan. Compper has simply not been good enough to hold down a place and that's been a big hole in the strategy.
There is no issue now about selling Boyata. We are out of the Champions League. There is a straightforward qualification round to compete in the Europa League, but Boyata should be out of the club as soon as possible now. We need to strengthen between now and the end of August. We are a couple of weeks away and this is usually when they begin to get their business done.
If we look at the starting XI who played in Athens, nine of them played under Ronny Deila. We are now in the fifth Brendan Rodgers transfer window and we really haven't added as much of as we should have during that period - of quality anyway.
Source: bbc.com