Neil Lennon's face told a story after Celtic's defeat to AC Milan on Wednesday. His words - "they did well but not well enough for my liking" - told us much more.

He was right to make the point, for his men have set monstrously high standards for themselves after last season's successes in reaching the last 16 of the competition.

In San Siro they lost two late goals when they could themselves have been ahead. The Scottish champions retained the ball for long periods and made Milan sweat, no doubt about it.

Yet they did not do nearly enough with set pieces and suffered from a lack of accuracy in their crossing throughout.

And there's the rub. The Rossoneri were under strength, missing a number of key players, but waltzed off into the Lombardy night with all three points.

Opportunities must be taken at this, the apex of club football. If not, there is a price to be paid.

It is an unforgiving maxim, but one which suitably describes the Champions League.

Bearing in mind they next host Barcelona before a double-header with Ajax, Celtic already have a mountain to climb if they are to escape the toughest of sections.

Lennon knows this, though he will not publicly put it in such stark terms.

One cannot help but feel that on the plane home from Italy those players and the coaching staff must have felt the ghosts of Gary Hooper and Victor Wanyama on their shoulders.

For this was a Celtic performance full of positives, and yet for all that doubt remains. Boiled down it is this: signings have been made - think Virgil van Dijk, Derk Boerrigter and Teemu Pukki - who may very well have what it takes to flourish at this level, given time.

Last season with the aforementioned Hooper, Wanyama and the also recently departed Kelvin Wilson, the side was tried and tested.

Those three players, each sold south to England, were an identifiable spine, adding sharpness, intelligence and brawn where needed.

Have they been sufficiently replaced? Well, it is often impossible to do so in a like-for-like manner.

The real point is that Celtic are, for the moment, a work in progress.

I also suspect that they lack strength in depth, perhaps in all sections of the pitch.

That is not a criticism of Lennon, however, as it is not always possible to bring first-choice transfer targets to Scotland.

That is a fact, albeit an unwelcome one.

My guess is that this year's green-and-white model will acquit itself just fine in Uefa's pre-eminent tournament without doing quite enough to qualify for the knock-out rounds.

They will likely end up in a dogfight with Ajax for third place and if that happens, and qualification for the Europa League follows, then there will be no shame in that.

In fact more game time against continental opposition, whatever their rank, would be a very good way for this young side to grow and develop before another crack at the big time 12 months from now.

Murray's surgery a tough choice but he'll come back stronger

It may not have been an easy decision but Andy Murray has chosen the right course in opting for back surgery which will in all likelihood keep him out of tennis for the rest of this year.

The Wimbledon champion has been troubled by a disc problem in his lower back off and on for the past 18 months or so. It has hampered his game at times this season, most notably in the withdrawal from the French Open back in May.

The calculation here is that the 26-year-old will be able to go under the knife - never a welcome prospect - and recuperate before heading to Florida for his traditional boot camp and then the Australian Open in January.

Thereafter he should be altogether stronger and fitter, ready for an injury-free assault on 2014 in which he will have the much-relished chance to defend his singles title at the All England Club next summer.

That may be some way off, but clearly Murray is thinking of nothing more than matching and then bettering a season none of us will ever forget. We will all wish him well.

Follow Douglas Beattie on Twitter: @alpha_beatt