Armstrong is under contract with Dundee United until 2016

Celtic are poised to reignite their interest in coveted Dundee United midfielder Stuart Armstrong.

The 22-year-old was the subject of a scouting mission by Parkhead assistant manager John Collins as United won their Scottish Cup clash in Motherwell on Saturday. Despite failing to take two scoring chances, Armstrong was the best player on the pitch.

Parkhead boss Ronny Deila, who is seeking between two and four new faces in the January transfer window, wants midfield reinforcements ahead of an assault on the last 32 of the Europa League.

Dundee United midfielder Stuart Armstrong (right) is being targeted by Celtic as a future signing

Armstrong features high on a list of potential transfer targets compiled by the champions, who have also had Tranmere’s 21-year-old Max Power watched.

First considered when Neil Lennon was manager, Inverness-born Armstrong is a former Scottish Football Writers’ Young Player of the Year and also featured on the PFA Scotland shortlist last season.

A product of United’s youth system, he is under contract until 2016 and is likely to command a fee in excess of £2million, albeit the Tayside club do not have to sell after restructuring their bank debt and selling midfielder Ryan Gauld and left-back Andrew Robertson for a total of £5m in the summer.

Meanwhile, Deila believes Scottish referees are of a better standard than any he has encountered in Europe this season.

Parkhead boss Ronny Deila is seeking between two and four new faces in the January transfer window

He was the beneficiary of a decision at Tynecastle last Sunday when John Guidetti was awarded a penalty by Willie Collum — although replays showed there was no contact from Hearts defender Brad McKay.

The Swede will face no retrospective action from the SFA — despite TV pundit Neil McCann claiming the striker had ‘conned the referee’ and had ‘thrown himself to the ground’.

But Deila, who insists his player did not dive, feels that errors of judgment will balance themselves out over the course of the season.

And the Norwegian reckons that, in general, mistakes from officials have been far less conspicuous in domestic games than in Champions League and Europa League matches.

‘The referees have been fantastic so far since I came here,’ he said. ‘It is a high level of refereeing in Scotland. It has been worse in Europe, like last Thursday against Salzburg. We have more problems in Europe.

Celtic striker John Guidetti goes down in the penalty box after Brad McKay's challenge

‘But the referees here have been good. Sometimes you get something for you, sometimes against you.

‘I don’t think a lot about it. If you perform well as a team, then you will win games.

‘We don’t want to cheat, of course we don’t. I don’t like that at all. I didn’t think this situation with John was like that.

‘The refs are trying to do their jobs as best as possible. Sometimes they have been right, sometimes wrong, but if you go through a whole season, it is going to be quite equal with decisions. The best team will win in the end.

‘I have found the refs to be approachable and very good. So far, I’m very satisfied with how they have been in Scotland. We have had more trouble with the refs in European games.’

Deila’s side take on Partick Thistle at Celtic Park tonight hoping to extend their lead at the top of the Premiership to three points.