If a substitute comes on and scores, everyone tends to notice, but there are other important impacts possible, as explained by Chelsea legend Pat Nevin in this week’s column...

It has become a popular refrain with Chelsea fans of late - ‘In Thomas we trust’. It stems not just from Thomas Tuchel leading us to the Champions League trophy, but also we must admit from him choosing some starting line-ups that have raised a couple of thousand eyebrows around the stadium. In the end, far more often than not, his thinking becomes clear once the games have begun.I recall against Zenit earlier in the season thinking, ‘Do we really still need Reece James and Azpi on the pitch right now as we chase the vital opener with 20 minutes to go?’ Moments later it is Cesar who whips in a cross that finally allows Romelu Lukaku to bring home the three points.

This turned out to be a vital decision that could easily be underestimated. Had we not squeezed in that goal, we could have been sitting with one point from six in the Champions League, a less than comfortable situation.The other thing the manager got right that night was the substitutions and the change in shape. As mentioned here before, Kai Havertz’s introduction developed the space for Romelu and six minutes after our young German came on, the net in front of the Matthew Harding was rippling.

Sometimes the substitutions and their effects are not obvious at the time. If the sub runs on and scores right away, we tend to notice it, but changing the shape of the game, the way possession is kept and the effect on the opposition can be just as important.Against Southampton we were having ‘one of those days’, well it looked like it could be one as the second half rolled on. The visitors had been brave, and certainly anything but negative turning up at Stamford Bridge and having a real good go at us. Granted we could have been out of sight by half-time, but from the referee’s first whistle in the second half, things changed and following the equalising penalty, we were under pressure.I was covering the game for BBC Radio 5 Live and even had one Chelsea fan complaining online that I was being harsh on the Blues for suggesting they were second best at one stage in the second period, but it was the truth, the whole truth and nothing but the truth, your honour. It had to be said, Chelsea needed to make changes and Thomas Tuchel after consideration did just that, to great effect.

First we had to get control of the midfield. Fortunately the boss had the best in the business at that sitting beside him ready and willing to do the job after a well-earned rest. Jorginho came on and it improved immediately. I mention not only a sub’s impact on our team but also his impact on the opposition and that was even more pronounced. James Ward-Prowse was so spooked by the idea of our Italian generalissimo controlling every movement on the pitch that he made a rash, uncontrolled tackle in a totally unnecessary position. Off went Ward-Prowse and the die was well and truly cast.Southampton were still capable of defending in numbers, but the important thing was to bring on some creativity and some vision. Hakim Ziyech was the obvious choice. Afterall creativity is his stock in trade, but he was overlooked. Kai Havertz might have been the next most obvious. Remember his contribution from the bench versus Zenit. But no, the manager thought Ross Barkley was exactly the man to seize the moment when it came.Ross didn’t score the goal and will not even get an assist next to his name, but his brilliantly pinged ball from the left-hand side to Cesar Azpilicueta’s toe inside the box was one of the best passes of the season from a Chelsea player so far. The trajectory and the pace were both exceptional. I still think Kovacic’s stunner of a through-ball to play in Lukaku versus Villa takes some beating, but it was a world-class pass. Cesar crossed it back into the danger area and Timo did the rest, even if VAR probably looked at it 100 times trying to find a way of denying Timo, as it usually does.

It was an unexpected substitution but also clearly the right one in retrospect. The other substitution that clearly worked was the introduction of Mason Mount with his indomitable industriousness. It was he who found himself at the byline before the short period of football pinball that followed. Once more Mason will not get the credit for an assist, that will be Azpi again, but he was the one who broke the lines and created the mayhem for Hasenhuttl’s men.It was also a big call to start with Trevoh Chalobah and I think we can agree that worked quite well, even before you consider his goal. Also, Ruben Loftus-Cheek and Callum Hudson-Odoi getting the nod was a minor surprise but once again, it was back to that mantra - ‘In Thomas we trust’.For all the excitement it was not an easy week for Chelsea after losing to Man City and to Juventus, even if we had the lion’s share of the game against the Italians. What we needed was a win to draw a line under those and even though it may have taken until the 84th minute, that is exactly what was done against Southampton.

Maybe the most important psychological boost was the weekend as a whole. With each of the other top-five teams playing against one and other, someone was bound to drop points and it was imperative we took advantage of that. The fact that all four managed to drop points was an unexpected but welcome bonus.So, it is time for another little break, for the club anyway, and maybe time to enjoy two factors. They say you should live in the moment, so at this moment Chelsea are champions of Europe and are top of the Premier League. Now if that isn’t something to sit back and enjoy for a week or two, I don’t know what is.