THE CELTIC dressing room is not giving much away, and that parsimony is not just restricted to goals.

Charlie Mulgrew sat in the media room in the wake of the 4-0 drubbing of Kilmarnock on Wednesday and was so unforthcoming one wondered if he had his lawyer present and was about to plead the Fifth Amendment.

The midflelder, of course, has no need to fear incrimination given he is a part of a Celtic side that is proving irresistible domestically. But can they prove to be invincible in the SPFL Premiership and the William Hill Scottish Cup?

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"We are not trying to think too much about records. We don't speak too much about it," Mulgrew insisted under questioning.

Fraser Forster has now racked up 1035 minutes without losing a goal and Celtic remain unbeaten in the league, so can a double be achieved without so much as a severe dunt to the champions?

There are two obstacles to a blemish-free end to the season. Celtic are so far ahead technically and in terms of depth of squad that they need only fear two factors: Morton mayhem or motivation meltdown.

The first is a reference to Celtic's only defeat domestically: the extra-time loss to Morton in the League Cup on September 24. One can never rule out a repeat, that is, after all, part of the magic of football, but there are solid reasons for believing the Morton mayhem has been replaced with a strong sense of stability.

First, the team that played Morton can never be picked again this season, most particularly because Tom Rogic is in Australia on loan. But it is also difficult to see Neil Lennon selecting a side for any forthcoming fixture that includes all of Nir Biton, Dirk Boerrigter and Teemu Pukki. All may feature individually, but not as part of a collective.

Lennon now has a solidity to his side that is bolstered with options in terms of replacements. Stefan Johansen, Amido Balde and Boerrigter all came on against Kilmarnock to make an impression, though the Dutchman's was restricted to a clean air shot when through on goal.

The nucleus of his side, though, is gaining in confidence and in accomplishment. Virgil van Dijk is a potential Dutch internationalist, Scott Brown, Mulgrew and Joe Ledley are too much for any Scottish team to handle in midfield and Celtic have flair and threat in Kris Commons and James Forrest.

Forrest, in particular, is a game changer. Commons has scored 21 goals this season and is the player of the year in waiting. But Forrest has the ability to break down defences with a sudden change of direction or a burst of pace. He used all of his traits to excellent effect against Kilmarnock.

The only deficit for Celtic is the lack of a quality striker with doubts of varying degrees over Anthony Stokes, Pukki and Balde. This is why a pursuit has been launched for Leigh Griffiths of Wolverhampton Wanderers. However, the striking problem must be put in perspective. This month alone Celtic have scored 16 goals without reply in the league.

The major problem for Celtic is how to maintain energy levels and urgency when the league is all but won. Elite sportsmen need that extra impetus that the fear of failure can bring to give their best performances. Last season Celtic faltered in the league, particularly in the wake of European matches.

Yet this season they have grown individually and collectively as the season progresses. There was much to admire about Celtic individually on Wednesday - Ledley and Forrest were outstanding - but it was the team's relentless industry which caught the eye. There was a moment just after half-time when there was the odd sloppy pass but the team clicked again to take the game far beyond Kilmarnock.

This may be Lennon's most intriguing achievement. How does he keep the motivation going, how does he encourage his players to run that extra yard, expend that extra moment of energy when all is won, when the result is rarely in doubt? "The manager keeps us on our toes," said Mulgrew, though he would elaborate no further.

The depredations of the transfer market - with Victor Wanyama and Gary Hooper leaving - may suggest Celtic are not as good a team as last year but the league table does not reflect this.

This points to the valid assertion that Lennon is a better manager than last season - certainly in preparing players for matches - though his team performed less well in Europe.

There may be a feeling in the Celtic camp that the disappointments in the group stages of the Champions League can be assuaged by achieving another domestic double, this one gilded by invincibility.

A glance at the fixtures before the break shows that next month Celtic only have to travel to Hearts, a ground on which they demolished the Gorgie side in their last meeting. St Mirren and St Johnstone visit Celtic Park in the league but, in the short term, the biggest test would seem to be the home tie against Aberdeen in the Scottish Cup on February 8.

Celtic supporters can be further encouraged by the realisation that any tilt at a double will not involve a clash with their nemesis. Morton are out of the Scottish Cup.