Nancy, Armstrong & Hearts v Rangers in spotlight

Another tumultuous week in Scottish football climaxes on Sunday with two big Premiership games.

But the big ones at the top are on Sunday as leaders Heart of Midlothian entertain Rangers, while Aberdeen visit a Celtic side in turmoil after Premier Sports Cup final disappointment was followed by a loss to Dundee United that consigned new manager Wilfried Nancy to an unprecedented fourth defeat in a row.

Here are some key things to look out for.

Just when it looked like Hearts' surprise title challenge was faltering, they have followed up four games without a win with two straight victories.

Combined with Celtic's sudden four-game losing streak under Nancy, albeit only two in the league, Derek McInnes' side are six points ahead of the reigning champions again.

Rangers are now only three points further back thanks to their two rivals' recent stumbles and a steadying of the Ibrox ship under their new head coach, Danny Rohl.

That makes Sunday's meeting at Tynecastle all the more significant for both their seasons.

Hearts' 2-0 win at Ibrox in September played a part in Russell Martin's Rangers reign ending after just 17 games - as well as cementing the Edinburgh side as potential challengers at the top of the table.

Having also beaten Celtic twice, becoming the first non-Old Firm side to win four consecutive top-flight meetings with the Glasgow duo since Dundee in 1985 would further strengthen their case, especially as it would widen the gap to third to 12 points.

Despite that tough run of November fixtures, Hearts have only lost once in 15 outings and remain unbeaten in 11 games at home, nine this season.

Like Hearts, Rangers under Rohl have managed to avoid defeats despite struggling to produce fluid performances.

The Glasgow side have themselves only lost once in eight outings and remain unbeaten in 19 away games in the league since defeat by St Mirren on Boxing Day last year.

Avoid defeat again on Sunday and it will be the seventh time in their history that Rangers have completed a calendar year without a league reverse on the road.

They head to Tynecastle unbeaten there in eight visits, winning six of them, since a 2-1 loss in January 2020.

Despite Rangers' deficiencies, Hearts remain the only side to have beaten them in the league this season.

Repeat their 3-1 win there on their latest visit in February and they will not only return to third spot, Motherwell having leapt above after beating Dundee on Saturday, but a three-way title race would very much be on again.

When Stuart Armstrong struggled for game time with Vancouver Whitecaps then was released by Sheffield Wednesday this summer after an inauspicious six months back in England's second tier, it looked like the 33-year-old's glittering career was on the wind-down.

It may be too late to resurrect hopes of adding to his 53 caps in the lead up to next summer's World Cup, but a return to hometown club Aberdeen has proved more fruitful for the former Celtic, Dundee United and Southampton midfielder.

Armstrong has provided three assists, plus one goal, in 17 appearances for the Dons, 15 of them starts and has become increasingly influential as Jimmy Thelin's side have improved their performances - and results - domestically.

He had not yet signed for the Dons when Celtic won 2-0 at Pittodrie in August, so this will be his first chance to face his former employers.

The four-time league winner with Celtic was rested as Aberdeen bowed out of the Conference League with a disappointing 3-0 defeat by Sparta Prague on Thursday and a fresh Armstrong could make all the difference as they look to cause an upset on Sunday in Glasgow.

Latest indications are that Nancy will still be Celtic manager when Aberdeen arrive in the east end of Glasgow come Sunday.

Whether or not he remains so could depend on whether they end the dismal four-game losing run since the Frenchman replaced interim manager Martin O'Neill.

Chief executive Michael Nicholson claimed that three staff members were assaulted after Sunday's League Cup final defeat by St Mirren, then it was announced that Peter Lawwell is standing down as chairman because of "abuse and threats" from disgruntled fans.

Celtic's latest four days of turmoil were topped off by the concession of an early lead as Nancy's side lost 2-1 away to Dundee United on Wednesday.

It might seem a bit unfair after just four games, but considering he has lost twice more since becoming the first Celtic manager ever to lose his opening two in charge, it is little surprise there was speculation that at least some of the board already want to end Nancy's tenure.

He is also the first Celtic manager to lose four games in a row since 1978.

Nancy can point to the fact the legendary Jock Stein was in charge at the time as proof that even the best can recover from such adversity, but the Celtic support are unlikely to be so forgiving unless the run ends on Sunday.

Lose again and it will be only the third time, and first since April 1953, that Celtic will have suffered five in a row.

Aberdeen arrive having earned more points than any other side in the Premiership since the start of October, winning seven and losing just one of their league games - and only losing twice in 11 overall.

However, Thelin's side are fresh from another bruising European defeat, away to Sparta Prague, that meant they finished second bottom of the Conference League as a four-game unbeaten run away from home came to an abrupt halt.

Nancy also has history in his favour, Celtic remaining unbeaten against the Dons in 32 meetings, winning 25 of them, since Aberdeen last won at Celtic Park in May 2018.

Since then, they have lost 10 of 12 visits, but much has changed since Brendan Rodgers led Celtic to a 2-0 win in the reverse fixture in August, with Aberdeen moving up to sixth and within five points of third-placed Rangers.

Thelin had his own shaky start as his preferred formation was initially found wanting in Scottish football.

Nancy, though, looks determined to stick with his change to three at the back rather than the four that served O'Neill well during seven wins in eight outings in interim charge.

It has left Celtic looking more vulnerable at the back, but Sunday's result could have as much to do with which side can take the many chances being created at the other end as they have the two biggest negative xG differences of any sides in Scotland's top tier this season.

Celtic have scored 26 goals from 33.5 expected goals (an underperformance of -7.5) while Aberdeen have scored 18 goals from 22.1 xG (an underperformance of -4.1).