KEVIN PHILLIPS' dismissal will naturally attract the spotlight, but it should not deflect from what really ended Saints' unbeaten start to the season.
During the superb rise under Gordon Strachan in the past two years, the many strengths of this current team have been well documented: phenomenal work-rate, cohesion, a superb defence and a striker who can't stop scoring goals.
They are attributes which have taken Saints to an FA Cup final, victories over Arsenal and Manchester United, a best Premiership finish and a record-breaking start to the season.
Yet for all those positives, there is a glaring and persistent weakness which, as long as it remains, will jeopardise Saints' lofty ambitions of breaking into the Premiership's top six.
You can call it imagination, you can call it flair or you can call it creativity.
Whichever, Saints do find it difficult to turn good possession into goals - especially when opponents defend in numbers.
Despite Phillips' equaliser against Steaua Bucharest, it was evident in the UEFA Cup last Wednesday, and it was evident again on Saturday.
Gordon Strachan admitted the final pass was lacking but, in the immediate aftermath to Saturday's 1-0 defeat, was somewhat defensive to the criticism that his team had not created chances, citing two efforts that were kept out by the post as well as the number of crosses and shots his team produced.
He did have a point - and his team perhaps just about deserved one too - but the statistics Strachan can't argue with are more long-term.
Despite finishing eighth last season, Saints managed only 43 goals in 38 Premiership matches - less than any other club which finished in the top half of the table.
So far this season, the ratio has been almost exactly the same with only eight from their opening seven league matches - and only James Beattie and Kevin Phillips have scored.
Saints do put in lots of crosses and do have plenty of shots (although often from range), but they can get predictable and do not create as much as they should, particularly from balls through the middle.
It was a deficiency which made it difficult to break-down Boro's central defensive partnership of Chris Riggott and Colin Cooper after Malcolm Christie had found Graeme Le Saux somewhat napping to cut in from the right and fire home what proved to be the winner after just 13 minutes.
In fairness, Boro created little more, with Saints' best first-half chance a 25-yard strike from Paul Telfer which just brushed the post.
Saints kept pushing forward and on the hour-mark Beattie was desperately unlucky not to be awarded a penalty when Danny Mills bundled him over just as he looked favourite to get on the end of a Fabrice Fernandes cross.
Neil McCann also went close, but his effort from a Beattie cross was just scrambled away by a combination of Boro 'keeper Mark Schwarzer and the post.
Then with just five minutes remaining, Phillips got on the end of a clever free-kick only to be denied again by Schwarzer.
Uncharacteristically, Saints' discipline was brought into question by the actions of Chris Marsden and Phillips. Both lashed out and, while neither reaction was really defensible, they were understandable.
Marsden was booked after raising his hand at Danny Mills and could possibly have been sent-off, although the former Leeds man had unnecessary barged into Saints' FA Cup final skipper after he had challenged Schwarzer for a loose ball.
Only 36 minutes had gone and while it is plausible Marsden was then substituted due to a need for more creativity, it is more probable Strachan could forsee further clashes with Mills and a possible red-card.
But he could do nothing to prevent Saints' first sending-off since Marian Pahars in the 2-0 home victory against Manchester City on October 5 last year - 44 games previously.
With just seconds remaining, Phillips was desperately trying to hurry a throw-in when Boro left-back Franck Queudrue cynically trod on the back of his leg.
Phillips instinctively kicked out and while his foot barely connected with Queudrue, the Frenchman went down as if he had been attacked by a mule.
It had no impact on Saturday's result, but Phillips' indiscipline will clearly cost Saints during his resulting suspension.
It was a frustrating end to what had been a highly frustrating afternoon.