To what extent are certain players "underperforming" because of the "quality" of the players they are playing with or maybe the tactics employed?

Let's look at Jozwiak for one. Started to get into his stride and was looking good when Bielik was in the side. Now he looks lost. As thers have said in other threads, language might be part of the cause. IMO he is certainly missing having Bielik around. He was looking better when we had that spell of playing om the front foot and pressing and harrying opponents instead of letting them do it to us. His slipping back has coincided with us going back to not doing those things. I still believe there's a good player in there.

Great goal against the Netherlands at the start of the season. Last night, 58 minutes in, Poland hadn't had a shot on target. On 59 minutes Joz was subbed on. With his first touch he crossed for their first goal. A minute later, with his 2nd touch he scored. 20 minutes later he was involved in a 1-2 that saw his return pass crossed in to Lewandowski to score.

He's also not helped by our opponents knowing he's good so, every time he gets possession he's double, triple and sometimes even quadruple teamed.

Add to that his having to spend much of his time back defending now we aren't on the front foot anymore and you have all the reasons he's not delivering for DCFC.