Photo: BBC SPORT
In truth, this was an FA Cup tie that neither side wanted. With Championship survival the priority for two of the league's smaller clubs, it was a match that seemed largely irrelevant and would only serve as a distraction. In particular, Harris nor Heckingbottom would have wanted a replay on a cold Tuesday night at Oakwell.
The Lions started the match as the slightly better side, getting some early shots off and winning a corner before they went behind in the 10th minute to Brad Potts' goal. Adam Hammill crossed in from the right and Millwall failed to deal with the ball, allowing the Barnsley man to smash a half volley past David Martin, who replaced Jordan Archer as one of six changes for the hosts.
Barnsley then had a spell of dominance and if they used the ball better in the final third, Harris's side could have found themselves 2 or 3 down by the half hour mark. Fortunately, Barnsley chose greed over teamwork at times, and Millwall remained in the game, allowing Aiden O'Brien to equalise ten minutes before the break. An excellent sliding tackle from captain Tony Craig allowed Jed Wallace to run down the left before crossing for the Irish international who slotted it into the net.
The goal acted as a turning point in the match. After O'Brien's equaliser, they were excellent, and Wallace should have put the home side in front before the interval but his poke agonisingly went just past the post. Millwall started the second half in the same fashion, and reaped their rewards just two minutes after the restart. A long ball forward was not controlled initially by O'Brien (who had his best game in a Millwall shirt for quite some time) but he eventually got it down before sending in a cross that just evaded the defender and fell for Ben Thompson to score his first goal in 21 months. It was an excellent finish by the academy graduate, and he may have given Harris food for thought with regards to team selection after this excellent display.
Wallace and O'Brien especially were causing the away team all sorts of problems at this point, constantly switching wings and moving the ball at speed. It was a horrible, late challenge on the former which put the Tykes down to ten men, and resulted in this game being over, realistically.
Steve Morison, who yet again led the line well for Millwall got himself more involved in the third and fourth goals. Morison's initial ball was eventually headed away by the defender before Wallace tried a 40 yard volley which was blocked and allowed O'Brien to score his second of the game. It was a superb strike from just outside the box and beat the goalkeeper comfortably.
The Lions were more than happy to see the game over the line at this point, but there was still enough about them for one more goal. Jake Cooper brought the ball calmly out of defence and played it into Morison, who flicked it through for Fred Onyedinma to curl the ball past the goalkeeper for his first goal of the season.
The last twenty minutes were easy for Millwall, with Barnsley trying to push forward but struggling due to Williams' sending off. They were able to defend comfortably and probably could have had more goals if they went looking for them, but chose to rest on the 4-1 lead.
“I thought we started the game well, we were excellent for 10 minutes and should have got in front but didn’t,” Harris said. “We conceded a sucker-punch of a goal and it took us 10 minutes to find our feet again.
“But the 20 minutes before half-time and the 20 minutes after half-time I thought was outstanding, as good as we’ve been this year. Some of our play was very impressive. We passed the ball well and moved well and looked a goal threat for a long period.
“I was delighted with the players who came into the team today and took their opportunity to show their quality.
“All the players, Tony, Ben and Fred, they all took their opportunity. I made changes to give people the chance for match minutes and also to rest players that have played a lot of football.
“When you do that sometimes you lose a bit of continuity in performances, especially here at The Den, but we didn’t show that at all today and we looked a quality side.”
Barnsley were reduced to 10 men in the 52nd minute when Joe Williams tackled Jed Wallace late, with Millwall winning 2-1 at this stage.
“At the time I thought it looked reckless,” Harris said of Williams’ tackle, which prompted a strong reaction from a number of Lions players. “The only reason it looked like a red card from where I was was because of red cards we’ve received this year, that the referee always wants to put down as endangering an opponent.
“It looked reckless and dangerous but when I played it wasn’t a red card. We’ve been on the receiving end of four this year and that’s the first one against us.
“I’d need to see it again, to see if it was dangerous or not.”
The fourth-round draw takes place on Monday night.
“In the nicest possible way, the cup is a distraction this year,” Harris said. “If we get to the fifth round then great, if we don’t then so be it and we go on to concentrate on the league. The league is the priority and that’s just the reality of where we are this year. We want to make sure we maintain Championship status and keep improving.
“It went very much under the radar last year that we beat three Premier League sides with three clean sheets. We’d like to have a crack at more of the top sides, but first and foremost we’d like to be here at The Den.”
"I was delighted we were at home today, I didn't fancy Barnsley away, not at all" Harris said. "In the next round I want to be at home, I want a big game at The Den.
"I'd like to have a crack at more of the big sides. I'd like to play Premier League opposition but if I had to pick one team I'd love it to be West Ham."