Pedro wasn't in an offside position. Still terrible defending.
No one was in an offside position because as I said, Gomez is playing them both onside. Mignolet should be on the far post ready to deal with the cross as it comes in, or save a shot from one of the two attackers. The whole reason it went in was because he was on his near post allowing himself the possiblity of being chipped. Even worse is that both Moreno and Klavan are on Willian as he crosses that ball and neither cut the cross out. Moreno is stood around watching whilst Klavan runs past him. Chelsea had two men in the box, we had six, plus the goalkeeper, and any half decent crosser of the ball is going to stick that on the head of either Alonso or Morata.
Look at the footage closely.
* 2 seconds in: Wijnaldum, Klavan and Moreno are infront of Willian. Matip, Gomez and Chamberlain are behind Alonso and Morata. Matip is playing them both onside.
* 3 seconds in: Matip runs towards the near post. He's seemingly unaware of the positionining of either Alonso (the nearest to him) or Morata. As an unintended cosnequence he plays Alonso offside.
* 4 seconds in: Klavan is at Willian's heals. Moreno has his back half turned in case the ball goes out wide. He should be closing down the angle for Willian's cross because if the ball goes out wide there's no immediate threat. Alonso begins his run. Chamberlain plays him onside.
* 5 seconds in: Klavan has run past Willian without puting a tackle in. Willian hits a cross. Moreno turns his back instead of closing him down.
* 6 seconds in: Alonso is now completely unmarked and in a position to nod the ball into the back of the net.
* 7 seconds in: Morata is now completely unmarked and in a position to nod the ball into the back of the net. Neither of that matters because Willian miskicks the ball which is now in the back of the net.
If we back up, what should have happened is this:
* The attack starts, Mignolet covers the back post.
* 2 seconds in: Liverpool's back line steps up and plays Alonso and Morata offside.
* 4 seconds in: Klavan makes a tackle on Willian while the ball is still outside the box.
* 5 seconds in: Assuming that's failed, Moreno should be putting in a second challenge on Willian.
* 7 seconds in: Assuming that also failed, both Alonso and Morata have had to move back to an onside position removing the threat of a cross. Assuming the ball is still miss-hit, Mignolet makes a comfortable catch and wastes a little time to run the clock down before releasing it for Liverpool to build another attack.
Six seconds of poor positioning and poor judgement lead to a goal that your average pub team would be ashamed of.