No one doubts Mourinho's abilities, as he is one of the best coaches in the world. In fact, every club Mourinho coaches has won trophies. Benfica is likely to win the first leg of the Champions League against Real Madrid because, in addition to playing at home, Mourinho is certainly a significant factor. He has been able to improve Benfica's performance and understands Real Madrid's game, having previously coached there.
The first leg will be hosted at Benfica stadium and they will have another advantage when Real Madrid won't have best player resource for that match by red cards (Asencio and Rodrygo). With experience of Jose Mourinho, Benfica will very likely be able take advantage of these things and turn them to a win at their home stadium then can play very defensive at Bernabeu stadium in a second leg.
Arbeloa is a young coach while it seems he does not have great management talent like Xabi Alonso and he will have very little chance of winning against Jose Mourinho in tactical fight between head coaches. He will have to rely to his star players for beating Jose Mourinho and Benfica but I am not optimistic with Arbeloa and Real Madrid's chance at all.
Remember in first meeting between Jose Mourinho and Xabi Alonso in UEFA Europa League semi finals, in a first season of Xabi Alonso with Bayer Leverkusen, he lost to Jose Mourinho and AS Roma. If Xabi Alonso failed, Arbeloa will have less chance to make something different and beat Jose Mourinho even he has many star players in Real Madrid.