How was it entirely his fault?
Multiple players failed for that goal.
Everything was under control until Lenihan decided to charge forward to try and press the midfielder. He left his man and once the ball was played past him it was a 4 vs 3 break. A chance on goal was inevitable at that point. He should have stayed with his man and conceded ground while McGree and others got back into position.
1) We have the situation under control. 4-vs-4 at the back, 3-vs-3 in midfield all our players goal side. Lenihan wins initial header.
2)Lenihan allows the man he is marking to spin and makes the poor decision to leave him and press the ball, despite McGree being in proximity to the player and despite the ball being fully 65+ yards from goal.
3) Lenihan is too slow to challenge and the ball is played beyond him. He has left Bola hung out ot dry, 2 vs 1, Howson and Dijksteel are exposed centrally as they have space behind to defend and two players to mark who are spread apart, so can't work together to cover them.
4) Howson is forced to move over to support Bola because Lenihan is now 25 yard out of position. Lenihan should be where Dijksteel is and Dijksteel should be marking their number 10. The key though is Lenihan is 25 yards out of position because he went steaming into midfield for a ball he was never going to win.
5) Dijksteel, Howson and Bola are all doing their jobs, but there is a free man from the 4-vs-3 break. Lenihan isn't even back in the box yet, on the edge of the frame. This was all preventable by Lenihan staying with his man and retreating in frame 2. It was a poor decision from an experienced player. Like I said though, I'll put it down to try too hard to impress on his debut. Without the ball this was his only mistake, with the ball he was a bit sloppy too.
I'm not sure how that goal can be blamed on anyone other than Lenihan, maybe McGree could have challenged better for the initial ball, but he was back to goal and it dropped away from him, the defender had the run on him. so he would always be second best to that dropping ball. I wouldn't class that as a rick, just got beaten to it. No one other than Lenihan had an error of judgement.