Phil Smith's verdict: Inside Sunderland's thrilling cup win and the welcome dilemmas Lee Johnson will now face

The Irishman had been suffering from cramp not long before and little wonder.

For ninety minutes he had been driving up and down the left wing, stretching the game and testing the opposition defence at one end, before helping his 19-year-old teammate against an impressive opponent at the other.

In those closing minutes there were Sunderland players out on their feet everywhere you looked.

Behind O'Brien Dennis Cirkin was completing 90 minutes for the first time this season. In central midfield Dan Neil had played not far off every minute in this punishing early-season schedule, while alongside him Carl Winchester was playing through a whack he had taken not too long before.

At right back, Niall Huggins had been thrown into action having not had any competitive football through pre-season.

Frederik Alves had gone off with cramp, another youngster playing his first competitive action for some time.

Though they too made many changes for the game, chasing an equaliser Blackpool had turned to some of their key threats, and Jerry Yates had already drawn one stunning save from Anthony Patterson when the equaliser came.

There was an error on Sunderland's part in gifting the ball away, and that fatigue had been all too easy as they struggled to recover. Tyreece John-Jules was able to drive into space and the finish for substitute Josh Bowler was a relatively straightforward one.

With just minutes to play, penalties looked inevitable and in truth, a draw would perhaps have been a fair reflection of a hugely entertaining game between two talented young sides committed to their progressive approach.

O'Brien, though, would get a second wind.

Huggins drove forward (again) down the right, gathering possession after some tidy play from substitute Elliot Embleton.

In front of Huggins was Jack Diamond, who had grown into the game as Sunderland ponder whether his short-development is best served by a loan move elsewhere.

There had not been a great deal of opportunity for him to drive at the opposition defence but through the second half his running power and pressing had been important.

Now the cross was inch perfect, and at the back post O'Brien was there to convert right in front of the jubilant travelling support.

Blackpool would have just enough time to drive forward once or twice more, but Sunderland were able to hold out.

Adrenaline up; more celebrations followed.

Part that unmistakable rush of ecstasy that comes with any late winner ('this is why we love football', Kyril Louis-Dreyfus would post on Instagram) and part appreciation for another front-foot display from this young team.

Neil Critchley said afterwards that the defeat had been harsh on his side and that was true to a large extent.

They had been a threat throughout, particularly in the wide areas where their speed and quality meant they always had the potential to push their opponents quickly onto the back foot.

They had started both halves the better, taking the chance to go ahead in the first but not in the second.

Statistically they just about had the edge in a superb cup atmosphere.

Sunderland, though, kept playing their way in even in the trickier spells of the game. They had clear cut chances in both halves, and appeared to be unfortunate when Nathan

Broadhead's fine finish was ruled out for offside midway through the first half.

All over the pitch, players left Johnson much to ponder with their performance.

At centre-back Bailey Wright was superb, the necessary foil to the young defenders around him. Defending resolutely, it is notable how much his play on the ball has improved now that he is fully fit and has some rhythm after injuries hampered his campaign last time out.

O'Brien has suffered from the club's general shift in style, where another playmaker is preferred to a second striker, but from the wide areas he excelled.

In central midfield, Dan Neil produced the best assist of the campaign so far for O'Brien's first, cutting the home defence open with a glorious through ball.

Time and time again he drove his side forward with his willingness to be brave in possession, and alongside him Carl Winchester was impressive.

A strong run of form at right back has seen his confidence clearly spike, and here he had the tenacity to match the technical quality.

The Carabao Cup is far down Sunderland's list of priorities this season but the significance of this win was clear.

Sunderland are building trust in their new philosophy both internally and externally, and there are few better ways to do that than with a win this thrilling, against a side who have twice beaten them in this calendar year.

What was most notable about that winner was its source.

It had been substitute Callum Doyle who had erred in possession for the equaliser, and so moments later the easiest thing to do would have been to play a long punt forward.

Instead he strode out, gliding past two before breaking the lines with his pass.

"I don't mind that error at all," Johnson said.

"Callum Doyle has got to be allowed to fail in those scenarios.

"He's having a hundred of those every game, and my disappointment really was that we didn't react quick enough to the giveaway.

"We have to encourage bold play. If you want to play like we do, you don't come in and criticise Callum Doyle for trying to cut the lines because he's done it 50 times in the five games previous.

"I don't mind that, but what I would like to see is us respond a bit quicker.

"To be fair, you can see how much the lads trust him in possession because we probably had a few too many ahead of the ball and that left us vulnerable."

A new era, indeed.

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