At a glance
Rampant Hearts surged eight ahead of Celtic at the summit of the Scottish top flight to strengthen their early-season title credentials and deepen the struggles around Brendan Rodgers' side.
Alexandros Kyziridis' fine strike and a spot-kick from Shankland gave the Hearts team a two-goal advantage that the title holders could not overcome.
Dane Murray - one of three youngsters in the Celtic XI - slashed into his own net in the early stages to put the home side ahead at a energetic their home ground, and it was the youngster's infringement that led to the spot-kick.
A stylish finish from McGregor of the season had swiftly equalized Hearts' opener, and Benjamin Nygren almost put Rodgers' team in front, choosing placement over power and allowing the Hearts keeper to make a crucial stop.
But the unstoppable Hearts side seized control again to consign Celtic to successive league defeats for the first time since 2023.
It also gives the Hearts a significant lead at the summit.
The previous occasion they were this far ahead was in the 2005-2006 season under their former manager, when they ended as runners-up - a rare instance that a team has not secured the title after building such an advantage in the last three decades.
McInnes, whose former team were runners-up to the champions on four occasions, has been at pains to play down championship discussions, as the club seek their first top-flight crown since 1960.
Rodgers has seen it all before at Celtic - both perfect and shaky beginnings to a season. He's yet to finish second best.
But on Sunday's evidence, this Edinburgh outfit showed they deserve serious consideration.
Hearts started with the confidence of a side unbeaten in the league this season - the popular attacker poked a cross in for the striker to stab wide.
The Glasgow team didn't help themselves with some uncharacteristic carelessness in possession and they buckled under their first real examination.
A headed effort from Braga was touched on to a post by Kasper Schmeichel and Shankland's return ball was diverted goalwards by the hapless defender, in for the injured Cameron Carter-Vickers.
Celtic grew into the contest and started stringing passes together. From one such move, the midfielder released the full-back to square for the captain, who slotted home.
Apart from the Nygren chance, the visitors found it hard to generate chances for striker Kenny or his substitute Yamada amid the absence of injured forwards their key strikers.
Hearts regained their intensity and adaptability in attacking from across the final third. Kyziridis, one of the finds of the club's relationship with shareholder Tony Bloom's Jamestown Analytics, struck low past the goalkeeper from the just outside the area.
Immediately after, the defender fouled Braga in the inside right channel and Shankland scored. And it could've been an more comprehensive victory, had replacement Ageu not been stopped by Schmeichel.
Celtic's second half was partially encapsulated in two incidents involving big-money recruit Engles.
The midfielder's wayward free-kick delivery was in sharp opposition to his European display in the Europa League win over their Austrian opponents and the national team player was then yellow-carded for a cynical foul.
Hearts head coach Derek McInnes: "You see how hard the players work, the togetherness. That's not sufficient on its own to win games week in, week out.
"What we have got, is good technical ability. You're seeing that from a lot of players who have been here before. Recent additions are keen to demonstrate their quality.
"We're in a positive moment at the minute, but it's only three points. It's crucial for us to keep winning. We've got loads to do, plenty to improve. Following the initial fixtures is normally an indication of where you are."
Celtic manager Brendan Rodgers: "The second goal was disappointing from our perspective. We failed to push up when the play was recycled as a line so when they play it, they should have players offside but didn't. We have got to do better with the attempt.
"Plenty of matches to play for. It's still so early. It's just nine matches. Obviously, being eight adrift is not what we would want. The positive is there's numerous fixtures to play.
"We now have to focus on the present and look to find more consistency and more quality in our play."
The Glasgow club are back at home on midweek against newly-promoted Falkirk (evening kick-off). At the same time in the top division, the Edinburgh outfit are at St Mirren.
A passionate gamer and content creator with years of experience in competitive gaming and strategy development.