Do substitutes have reduced impact?

[Myspip]

Sometimes I play friendlies with a starting eleven with terrible attributes and around half time swap in my main eleven (about 150% quality). In the last few days, my better team has failed to score more than. 1 goal against the competition in the second half, even if I regularly would beat those teams by 6-0 or more when my best team is used for the entire game. Has anyone else experienced something similar?

[KenoticFC]

This is an interesting topic and all I can do is give you my speculation. The answer is no, I always felt that the starting formation is what determines 90% of the match, I have always believed in some kind of predestination.
Subs do have an impact and I can assure you from experience on important matches, the trick is you have to have some good ones and of course your best team should be on the pitch.
Last season I tanked and in some matches I would reverse my formation back to normal during the second half. I did not notice a lot of changes, maybe some goals were scored but it felt like the original set up was what determined the match, even if the other team was weaker. If you start weak, it is very unlikely to go strong again. This is why the best subs are when a player is not performing as usual usually even a lower rated sub can come in and have a better impact. Also when one of your strikers is not scoring. Usually I would advice to not touch the defenders unless there is an injury or an early yellow card.