As far as English clubs are concerned, it is only Liverpool and Manchester City who are being talked about as genuine Champions League contenders this season.

What a source of frustration that must be at Manchester United, the three-time European champions.

Jose Mourinho would love nothing better than to secure another triumph as he did with Porto and Inter Milan, yet there has been notably little talk of the Red Devils challenging.

The ignominy of last season's exit at the hands of Sevilla has played no small part in that and little has really changed since then, with Fred the only significant signing of the summer transfer window.

Rio Ferdinand is among United's doubters and he has pinpointed their defensive frailties as the reason his former side can't expect to progress all that far.

Ten years on from being a part of the side that secured glory in Moscow, Ferdinand was asked whether he thinks United are ready to challenge again.

"No, they aren't and they won't be," the BT Sport pundit said, via the Daily Mail.

"They haven't bought anyone in who is going to change that. They've got a Superman [David de Gea] in goal. If he's not in goal I don't know what would've happened over the last four years. The guys in front of him, there are inconsistencies every week.

"You can't build a defence on people that aren't going to be reliable. And then when people are in there they haven't performed.

United's problems at the back

"Has [Victor] Lindelof worked? No. Has [Eric] Bailly worked? No. [Chris] Smalling and [Phil] Jones haven't kicked on, given the potential they showed when they first came through the door."

The back four will be pitted first against Young Boys on Wednesday night, before facing Valencia and then a very familiar face in the Juventus attack.

The thought of United's often haphazard defence up against Cristiano Ronaldo ought to be keeping Mourinho awake at night, but he wasn't permitted to do anything about it in the summer.

Having started Smalling and Lindelof together in consecutive league games, it appears the Portuguese has at least now settled on his preferred partnership and that is a step in the right direction.

Until United start putting in some solid defensive displays on a regular basis, however, few are going to take them seriously again. 

Do you think United can challenge in Europe? Have your say in the comments.