THERE was a time when, by this stage of the season, you could rely on Celtic to have embarked on a relentless run of form. For as long as the league title was in any doubt, they would heap the pressure on their rivals by rattling off a series of victories produced by their superior firepower.

Well, here we are in April, and the league title is indeed in doubt, but the champions are still stuttering their way through games. Their goalless draw at Dens Park on Tuesday night put them five points clear of Aberdeen with six games each to go, but they have had few commanding performances. By rights, with the resources at their disposal, they should have been out of sight by now.

For the fact that they do at least have that lead, and remain favourites to retain their title, they largely have one man to thank: Leigh Griffiths. Of course, it’s good news for Scottish football in general, as well as for Celtic, that they have a striker who is steadily becoming the master of his art. But there is a clear danger for the club in what has become an over-reliance on Griffiths, and it is a problem they do not look like overcoming any time soon.

Traditionally, Celtic would bring through some of the best young talent in the country season after season, with whole teams composed of players born and bred in Scotland. That tradition has been carried on to an extent by the emergence of Kieran Tierney this season, but in recent seasons Celtic have pursued a policy of recruiting continental players who can mature in the Premiership, contribute substantially to the club for a season or two, and then move on to higher things. With the likes of Fraser Forster and Virgil van Dijk, sold on to Southampton for £10million and £11.5m respectively, they have got their money’s worth while the players were with them and then traded them in for a handsome return.

More recently still, however, their activity in the transfer market has been less sure-handed. For instance, at 32, Carlton Cole is on the way down rather than up, while 29-year-old Colin Kazim-Richards has also been unimpressive. Celtic did not have to pay a transfer fee for either, but that cannot be any consolation given they have contributed so little.

At 19, Patrick Roberts looks set to have an excellent career ahead of him - but not at Parkhead. He could do a lot for Celtic in the short term, but at the end of his 18-month loan spell he will return to Manchester City, who will reap the benefit of his apprenticeship up here.

Include players such as Scott Allan, Stuart Armstrong and Gary Mackay-Steven, and the signing policy looks even more confused. Most concerning of all for the club is the fact that, somehow, they have failed to sign a striker who can be relied upon to share the goalscoring burden with Griffiths.

The 25-year-old is out on his own in the Premiership goalscoring charts on 26, with no-one else having yet scored 20. The next Celtic player is Tom Rogic, who has yet to get into double figures. Griffiths’ tally is all the more remarkable given he has been playing on his own up front, but even someone as gifted and dedicated as him cannot be expected to score in every game. And when he draws a blank, Celtic often draw.

Still, it is a heartening fact that a man once seen as one of the game’s more erratic characters has matured into one of its most dependable players. And, for those who watched him in his younger years, it was always a question of when he would come good, not if.

“He’s a hard worker, a team player, and he’s always had a great shot on him,” said Billy Brown, who was assistant manager at Hibernian when Griffiths was there. “He always had goals in his locker, and as he’s matured as a person he’s matured as a player as well.

“I never had any doubt about him. I always thought if he gets the right platform he’ll score goals. I maintain that if Leigh played up front at Celtic with another striker he could score 60 goals in a season.”

Dundee’s James McPake, who captained Griffiths at Easter Road, has also been impressed by his former team-mate’s development. “He can play up there on his own. He’s developed physically as well - I noticed that the last couple of times I played against him. He’s a lot stronger, so he’s got no problems if he’s asked to play that way.

“He’s been the one scoring the goals. As long as he’s fit I don’t think they’ll have a problem, because he always will score goals.

“The biggest problem is if he gets injured. But that’s Celtic’s problem.”

It certainly is.