Lol, and what the hell is Semedo supposed to? Just leave the player and start running after the overlap? Once the ball is passed, he cannot run faster than the ball and get to the runner for gods sake.
Yea seems like Semedo should have just cleared the way for Guedes to enter the box and score. On his right foot too. If Semedo moves to cover the overlap, Guedes gets in the box in a more dangerous situation, as Rakitic was very late to react. As it was, Gaya still had to place a perfect cross and Umtiti to make a mistake for them to score. Gaya did well there, normally that's not a critical position to concede. It's a crossing position but it's not immediately threatening, if we do our job. So even with Gaya there with the ball, we could have done well to block the cross or block Rodrigo. Umtiti and Vermaelen both make mistakes there in the box, and even Stegen could have maybe done more as the cross was quite close to him (although it was struck with some pace).
Everybody can tell you that the central zones are considered the most dangerous. You cover that first, even at the expense of leaving the outside areas free. You always block the easiest access to the goal, and that's what Semedo did there. Stuck with Guedes in a 1 vs 1, one of Valencia's most dangerous players on the ball.
Don't agree at all with the notion that fullbacks need to leave their man (the opposite team's winger) alone so they can block a "potential" pass to an overlapping fullback. In 4-4-2 it's the midfielder's job to track the runs of attacking fullbakcs.