I agree with you that currently the squad led by Jose Mourinho is still not strong enough, they still have to bring in a few more players to complete positions that have so far been considered to have not given their best performance. True, there is no doubt that Jose Mourinho is a coach who cannot be doubted about his abilities, but still it must be supported by players who can implement their strategies properly.
The first leg of their match will be played at home to AS Roma and in general AS Roma have not met Bayer Leverkusen in any competition for a long time. The last time they met in a match was back in 2015 in the Champions League group E which on a H2H basis for now is still almost even for these two teams. But for this season I have more confidence in the strategy that will be developed by Jose Mourinho because they really want to be in the Europa League final this season in order to win a trophy even though they have to face even tough opponents.
The strength of these two teams is relatively balanced but I am sure that Roma will be able to win the 1st leg which will be held at the Olimico. The Jose Mourinho factor is clearly more of a consideration for the Giallorossi, he has qualified experience in Europe but it also depends on the Roma squad in which the match is played, meaning that none of Roma's mainstay players are seriously injured.
Roma's seriousness was seen when they played the second leg when they were able to beat Feyenoord into extra time. In normal times, Mourinho's men almost had to bury their dreams, but The Special One was able to motivate his troops to catch up and force their visitors to recognize the greatness of Roma. Alonso certainly has to learn from the number of games played by Roma, otherwise he will become the next victim of Mourinho's cunning.