Tonda Eckert’s side are now 17 games unbeaten in all competitions, with vital contributions from both of their signings – Daniel Peretz and Cyle Larin.
Israeli international Peretz is a much-needed upgrade in goal, while Larin is a much better tactical fit for Eckert than even top scorer Adam Armstrong.
The squad size was reduced with unwanted players moved on, while James Bree, who returned from a loan at Charlton, is a consistent top performer.
All of that has helped Saints to climb from 15th in the Championship on January 17, 12 points off of Hull City in fifth, to overtaking them this weekend.
But there is another strong contender for the best bit of business Saints did in the window, having dismissed interest in one of their sellable assets.
Taylor Harwood-Bellis scored the winning goal from a Ryan Manning set-piece against Derby, in what was an all-round Man of the Match performance.
Minutes after his sixth notch of the season – only Finn Azaz and Leo Scienza have more – the 24 year old made a crucial block to deny Carlton Morris.
In all, no Saints player made more defensive contributions than Harwood-Bellis, who had the best duel success rate in a game defined by battles.
Harwood-Bellis, elected to wear the captain’s armband when Jack Stephens does not, has played more minutes than any Saints player this season.
Taylor Harwood-Bellis heads Saints in front against Derby (Image: Stuart Martin)
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West Ham came in with a bid worth £12million for the Man City academy graduate towards the end of the January window, and it was swiftly rejected.
The club valued the defender, whom they paid £20m for in 2024, significantly higher, and were very reluctant to lose him halfway through the season.
The decision is already paying off in points, and Saints will hope it can lead to a much more lucrative prize of promotion back to the Premier League.
Harwood-Bellis had a troubled start to the campaign and was twice dropped by Will Still, and made more individual mistakes last season than anyone.
But he is the only player to have played every minute for Eckert.
He became a father for the first time in December, which the head coach believes has offered the one-time England international a new perspective.
“It’s very nice to see him growing, because I think he’s only getting better week by week,” said the German, speaking after the win against Derby.
“I think he’s a completely different player from what he was at the beginning of the season, and we’ll do everything to support him to continue on that.”
Harwood-Bellis has himself praised Eckert, saying: “The manager has given me so much tactically, even little adjustments to my positioning.”
The responsibility to clear the lines falls to Harwood-Bellis more than any other Saints player, but so does the requirement for controlled possession.
He leads the side for accurate passes and long balls per 90 minutes, and his defence-splitter to Tom Fellows against Oxford showed the best of his range.
Harwood-Bellis now routinely attracts praise from national media who attend the odd game, and he looks destined for another crack at the top flight.
Without him, had he been wearing a claret and blue shirt recently, it is fair to say that the Premier League would be much further away from St Mary’s.