THERE has long been a theory that it is one thing to play well against the Celtic jersey - it is quite another to play well in it.

Many have fallen foul of this test, winning a move to the Hoops on the strength of eye-catching performances when competing with their current stars, but failing miserably to cope with the unique pressures playing alongside them involves.

Gary Mackay-Steven and Stuart Armstrong are the latest to be challenged to show they can replicate what so impressed Ronny Deila when he watched them slug it out with his players while wearing the tangerine of Dundee United.

Both have passed muster with ease. In fact, they came out of the blocks flying, both men scoring on their Celtic debuts against Partick Thistle.

Five games and less than four weeks on, however, the brakes are about to be applied,

Armstrong picked up a glute muscle injury in Sunday's demolition of Aberdeen, which makes him a doubt for tonight's game against St Johnstone.

And, after this evening's match, the electrifying pace of Mackay-Steven is also about to be unplugged for a couple of weeks.

The dynamic duo are ineligible to play for Celtic in the domestic cups, having already appeared for United in both competitions.

To add their frustration, the pair will be reduced to spectators for two of Celtic's most important games of the season, Sunday's Scottish Cup quarter-final and the following weekend's League Cup final.

And, as luck would have it, they are both against the team they have just left, Dundee United.

Mackay-Steven is doing his best to be philosophical about it, and wants to do his bit to make sure his team-mates go into this weekend's cup tie at Tannadice still riding the high they climbed upon as they demolished Aberdeen on Sunday.

That will require another victory against St Johnstone this evening, a result which would allow the Hoops to move into cup mode with a nine-point cushion over the Dons with just 11 games to go in the title race.

From a personal point of view, he also wants to leave his mark to ensure that whoever steps into his position in the next couple of games is fully aware he plans to be back in contention when eligible again for the league game against his former club on March 21.

Such has been the instant impact he has made, Mackay-Steven is once again being spoken about in Scotland terms.

But he is not about to fall into the trap of believing he has made it as a Celt.

As he quickly pointed out, Mackay-Steven is reminded every day in training what is required to retain his coveted place.

"Since my first week of training here, I have realised the stature of the club, and that the quality of player that is here is very high," he said.

"I knew it would take a lot of hard work on the training ground to, first and foremost, get in the starting XI.

"But, I am confident in my own ability.

"I have worked hard in training, and, since the manager gave me the nod, I have tried to bring that into the games."

He has done so successfully, even when the opposition has been of the calibre of Inter Milan and the arena the San Siro, where he showed he had the temperament and confidence to go at the Serie A superstars.

That particular European adventure is now over, but there is still plenty to keep Mackay-Steven excited, as the smile and the wide-eyed expression betrays.

He will treat every one of the dozen games for which he is still eligible this season as an opportunity which must not be passed up.

And, if he is disappointed that the league is the only part of the Treble which he can help try to deliver, he hides it well.

"I knew before I came here that I was cup tied and that I'd be missing these games," he reasoned.

"It's never nice. I remember when I first signed for Dundee United, we had to play Dundee in the cup, and I had to sit in the stand.

"I never like watching. But, that's out of my hands.

"Rules are rules, and I will support the lads and just look forward to the games which I can play in.

"I will keep working hard on the training ground because the depth of squad is deep.

"So once you get your chance, you have to be ready to take it."

Mackay-Steven and Armstrong could still have an important part to play in Celtic's quest to record a cup double over United.

Their in-depth knowledge of the strengths - and weaknesses - of their former team-mates will be an asset in Deila's detailed analysis of the last team to beat the Hoops in domestic competition.

Since that 2-1 defeat at Tannadice on December 21 - Armstrong scoring what proved to be the winner - the form of both sides has moved in completely different directions.

While Celtic have won their last 10 games domestically, shorn of two of their best players, United have lost three of their last four, and only drew the other.

Not the kind of build up anyone would want ahead of three games in three weeks in three competitions against the Hoops.

But Mackay-Steven warns against any complacency and said: "I know first-hand the talent of the players who are there, and of the manager.

"They have got a lot of good, young players.

"They have not had the best of results lately, but I am sure a great performance is only just around the corner for them, too.

"So they will be a big threat to us."