Back 4 of El Hamed, Julien, Ajer and Hayes looks the most likely scenario as Taylor looks a major doubt, Boli's out in the cold and I doubt Lenny will risk going with a back 3 away from home in a European tie.
On the subject of Ajer......I've been saying this since BR put him there I don't think the bhoy is a CB he's a DCM who's being crowbarred into a position that's not suited to his skill set. This was a flawed BR experiment borne out of necessity and the fact Lenny still has to continue with it is a damning indictment of our ongoing failure to properly address our longstanding CB issues.
I'm not saying Ajer is an awful CB, he's not, he's been decent enough considering it's not his natural position but like others have pointed out above he has some major flaws as a CB which have only been highlighted even more since we've started playing 3 at the back and I'm sorry but 'decent enough' isn't an acceptable approach for a club like Celtic. It should go without saying but what we need is proper CB's playing at CB and proper CM's playing in CM.
Add to the fact that there have been times (like yesterday against the sheep and in both Sevco games for example) where we've been crying out for a more commanding figure with height and physicality in central midfield yet we're having to play the guy who has all of that at CB. It's a nonsense for a club our size.
Get the bhoy back playing in his natural position and sign a proper CB.