We really don't need as many players as possible. Europa league only adds a few more games compared to the CL (assuming we even reach the final) and youngsters can play in the earlier matches. Speaking of youngsters, there's always people on here crying out for the kids to be given a chance. How are they going to be given a chance if we have a massive squad? Jose has never been the kind of manager to massively rotate his teams, which is why he prefers a squad of around 23.
Mourinho has also never been in the Europa League (as far as I'm aware), so he needs to enhance his squad in order to accommodate for the extra games. It plays more of a mental strain on the players who play in both the EL and league games due to the Thursday - Sunday schedule. Yes, it's the same amount days between European and domestic games, but there is a certain demotivation to playing after all your rivals especially if they have picked up their points.
Mourinho was very vocal about the nuisance of Europa League and that it shouldn't really be seen as much of an achievement when he took over at Chelsea second time round, however I got the feeling that he may have been echoing Ibramovich's opinions on the competition rather than his own.
In order to compete in both Europe and the Premier League, teams now more than ever, need a squad with good depth. It's also a great opportunity to keep morale high, as players who would most likely have a back up role for most the season have an opportunity to play 90 minutes of first team football and prove that they merit more game time. Not to mention youngsters having the opportunity to play first team football to stream line their transition to the first team squad.
Who would replace Valencia if he was out for the rest of the season? Darmian can cover both sides of defence, and has only had one season here in which he produced some stellar performances. He fell off big time as the season progressed, but so did most players. He deserves another season.