Former Celtic skipper Paul Lambert has admitted he fears the current Hoops squad is not big enough to compete on all fronts this season.

Celtic slipped out of the Champions League after a third qualifying round defeat to AEK Athens on Tuesday evening, after which manager Brendan Rodgers made little secret of his frustration over the club’s summer recruitment to date.

Asked about Rodgers’ post-match comments, in which he pointed to Liverpool’s spending since their run to last season’s Champions League final, Lambert told BBC Radio 5 Live: “I don’t think Brendan said anything derogatory to the board or anything, I think he’s just stating what every manager wants to do, strengthen their team with… one or two players that come in and give you a helping hand.

“That’s the big thing, you want people to come and strengthen to give you a hand because you have got so many league games, cup games, European competitions, you need a big squad.

“Looking at Celtic at the minute, I’m not sure they’ve got a big enough squad to go and compete on all fronts.”

With Kristoffer Ajer suspended, Rodgers’ resources were depleted further by the absence of fellow defender Dedryck Boyata, who has been linked with a move away from the club.

Boyata posted a video of himself undergoing treatment ahead of the game, although his manager later insisted the player was fit.

Lambert said: “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’.”

Celtic still have an opportunity to make it into the Europa League, although having lost back to back games for the first time under Rodgers – they went down 1-0 at Hearts in the league at the weekend – their difficult week has done little to quell the optimism of arch-rivals Rangers, who are unbeaten in seven competitive games under new manager Steven Gerrard.

But Lambert had a warning for the former Liverpool skipper.

He said: “He’ll be judged by the Celtic games, he’ll be judged by that. If you don’t come away beating Celtic, then second is failure up in Scotland.

“I’ve been in that position myself, I know exactly what it’s like. It doesn’t matter if you win seven or eight games in a row, your big yardstick is Celtic and if they can get above them and they go and do that and win the league, then it’s a success.

“Anything less, trust me, will be regarded as a failure.”