Saints marched on after a lacklustre first half to dominate the second and in the end cruise to an easy victory, that could have been 5 or 6, now they will get the chance to test themselves against Premier League opposition as they travel to Liverpool in the next round.
As expected Russell Martin rang the changes, but this time instead of the 9 changes in the first game up at Watford, he made just 7 from the side that beat Rotherham on Saturday for the replay, keeping much of the spine intact with Jan Bednarek, Flynn Downes, Will Smallbone and Che Adams staying in place.
Truth was the first half was a little disappointing, although Saints dominated and had the better of the chances, it was still 0-0 at the break.
Sekou Mara was one who looked lively though in the first half and created a great chance for Che Adams and in the second half he didn't take long to open the scoring, firing home in the 52nd minute and then 6 minutes later doubling the lead with a scorching shot that had the crowd on it's feet.
Now the game had swung firmly in Saints favour and Watford seemed to know it, Saints kept the ball, they passes and they swaggered and the crowd were enjoying themselves now.
Russell Martin again didn't try to make big changes until the game was well and truly won, prior to the third goal by Che Adams in the 76th minute (Perhaps a good omen here) the only change had been to take off Tyler Dibling to a standing ovation, the youngster had shown that he had pace, he had strength and he wasn't awed by the occasion or the opposition.
He reminded me a little bit of Matt Le Tissier, a little bit of Jack Grealish, but with strength and pace.
After the 3rd went in it was a triple change on 77 minutes and now Saints truly gave Watford the run around as their fans headed for the exits.
It could have been even more in the final 10 minutes or so, both Sam Edozie & Sam Amo-Ameyaw hit posts and several other chances could have gone in.
So it is easy to wax lyrical about some of the youngsters and we have to be careful to not get too carried away, some will say it was only Watford and they too had made changes to their side, but this was also a side with changes for us and our youngsters were up to the challenge, all three goals were scored before we brought on the big guns.
Jayden Meghoma looked the part at left back and slotted in well, Tyler Dibling I have already covered, he could truly play a bigger part in the Championship this season Shea Charles looked strong at right back and then there was Sekou Mara.
Mara has had some stick this season and many forget that he is still only 21 and learning his trade, today he showed he has listened and learned and he too staked a case for more game time, he took his too goals very well and created several chances for others that could have been put away.
He looked strong and he looked confident, he is a good asset to be able to bring on in the Championship late in the game.
Sam Edozie returned from injury and showed he too is ready to show that he has a lot to offer going forward, Sam Amo-Ameyaw looked eager to show that he is pushing for a chance in the League again at some stage.
So 24 games undefeated and apparently now a club record, not just since joining the league but in all time.
So we march on and although I do offer a word of caution here, it was still a result against a side in the same division as us, it wasn't a side two leagues below, our youngsters put in a shift against those they may well face later in the season in the League and came through.
But now we face the biggest test of the season, a trip to Anfield to take on Liverpool and a daunting task for any Premier League side, let alone one from the Championship.
Russell Martin will have a dilemma, does he stick with his strategy of changing the side for FA Cup games and to be frank the trip to Anfield comes smack bang in the middle of a run of games that starts on Saturday 10th Feb against Huddersfield and ends at Leicester City on 16th March where including Liverpool we play 9 games in 5 games.
We will find out on Wednesday 28th in what is now a crucial part of the season where we will truly find out where this season is going.