Overall he played very well on Monday. But there were a couple of times when Ibe went past him as if he wasnt there. Partly because he got his body position wrong, partly because he hesitated over whether to tackle with his right or left foot. It would take him many, many games to get comfortable in that role. Its why Clichy has looked very uncomfortable when played on the right, despite being born right footed. Its why when Zabaleta has played on the left he's looked half the player, always trying to force the attacker inside so that he can tackle with his right foot. And Fernandhino would face far better attackers than Ibe.
Apart from which, it seems that the main reason for playing him at full back is so that Yaya can continue as the single pivot. Overall that's worked well so far, but much less so when teams have pressured him. Yaya as the pivot and Fernandhino as left back provides our opponents with two very obvious options for applying pressure.