IF first impressions count for anything then the signing of Stuart Armstrong and Gary Mackay-Steven could well prove to be one of Celtic's shrewdest investments for quite some time.

For a combined outlay of around £2m - less than it cost to recruit Stefan Scepovic in the summer - the Parkhead club was able to prise the pair away from Dundee United on the final day of the transfer window after a spot of late-night bartering.

Ineligible for both the League Cup and the William Hill Scottish Cup, both players had to wait until last night - nine days after signing - to make their competitive debuts.

Finally unleashed, they did not disappoint, both pivotal as the Parkhead club moved three points clear at the top of the SPFL Premiership on the back of a comfortable victory.

Celtic have always been a club that has cherry-picked the best of talent playing at other Scottish teams but it is a source that has largely dried up in recent years. Given the immediate impact made by Mackay-Steven and Armstrong, they may not leave it so long to go back into the domestic market for another rummage around. A goal for each man embroidered hard-working, impressive performances.

Those who scoffed that the pair would be no more than bench fodder at Celtic were soon eating their words. Both played from the start at Firhill - an injury to Kris Commons perhaps made Ronny Deila's decision easier - as part of the three-man attacking trident supporting striker John Guidetti.

Mackay-Steven was on his usual right wing beat, with Armstrong lining up on the left, the pair either side of Stefan Johansen. Both showed the traits that had made them such hot properties at Tannadice - Mackay-Steven embarked on a number of meandering runs, Armstrong demonstrated vision and touch in spades - but there were other qualities to admire too, both showing a willingness to track back and fill in defensively whenever required.

When one Mackay-Steven run forward ended in vain - the winger in his haste having collided with the advertising boards - the travelling support was quick to show their appreciation for his efforts, and again when he finally jogged off after 82 minutes, his work done for the night.

Of course, the easiest way to win over any new fanbase is to score a goal and both players managed it within the opening half hour. Few debutants if any, in fact, could have done so as quickly as Mackay-Steven.

His run into the box from kick-off earned Celtic a corner and, when Thistle failed to deal with Johansen's delivery, the winger was on it in a flash to rifle a shot past Scott Fox, making his first appearance in the Thistle goal for three months. Only 38 seconds showed on the stopwatch.

Armstrong would have his moment, too, soon enough. Mackay-Steven was involved in that one too, capitalising on a Callum Booth mistake to cross for Armstrong who in turn fed Guidetti. The Swede could have taken it first time but instead took a touch and played it back to Armstrong who thudded an effort past Fox, making a mockery of the idea that new signings all need time to settle at a new club.

Johansen, who linked well with both new players, would not be denied either, fizzing in a low show after 66 minutes to confirm the Celtic victory, not that it was ever in any real doubt after Mackay-Steven's early strike.

Only Guidetti of the front four failed to score, the Swede coming closest with a flashing volley wide after being played in by Johansen. He is still without a goal to his name since the end of the November.

Guidetti's personal frustrations aside, this was as comfortable a night for Celtic as could have been imagined. It is hard to believe that just a month ago there were some questions over their domestic supremacy as Aberdeen threatened to topple them from the top of the table.

Celtic's response has been to win seven successive matches over three competitions to reach the League Cup final, the quarter-finals of the Scottish Cup and to move three points clear at the top of the league with a game in hand.

It is difficult to see just who, domestically at least, has what it takes to stop them, although Internazionale next Thursday in the Europa League will surely present a stiffer challenge.

Almost forgotten amid the rush to praise the two debutants was the fact that the Celtic defence had racked up another clean sheet. It is now 745 minutes since their goal was last breached - by Armstrong, ironically - and beyond a late Gary Fraser shot it was difficult to recall goalkeeper Craig Gordon having too much to do. St Johnstone will be next to test Celtic's defensive resolve on Saturday.

This was a disappointing night from Thistle's perspective and the more pessimistic among their number may fret that they are not entirely clear of the relegation picture just yet, as a record of one win from their last nine games would seem to suggest.

They showed promise in patches - in particular at the start of the second half - but never really recovered from the loss of such an early goal, the biggest cheer afforded to Dennis McQuade as the 1971 League Cup winner made the half-time draw. From Celtic's point of view, though, this was a night for hailing new heroes.