Lampard made it pretty clear the direction he saw for the team, a lot of emphasis on utilising younger, accademy players.
Was pretty obvious from early on that Rudiger in particular was out of favour with Lampard and it doesn't take a genius to work out we don't need 3 senior left backs in the squad.
So over to senior management to slim down the squad size because you don't need 5 centre backs and 3 left backs. It was a failing to move on one of each from those positions.
I would argue we have too many central midfielders too, especially as Lampard was making a path for Gilmore into the first team squad but we did allow Barkley and RLC to go out on loan so that wasn't as bad.
Our squad is too big. That's not an objective opinion it's pretty much fact. Doesn't matter who the manager was/is, there is absolutely no way you can keep that size of a squad happy.
The problem Lampard had is that he goal/vision whatever you want to call it was to entrust and promote younger players. So as soon as there are a couple of poor results senior players objections start to become a little bit louder and agents will start to be in the ear of certain people in and around the club.... It's just the way things are in football,
To say Lampard "didn't know how to deal with senior players" is a frankly (no pun intended (well maybe a little intendeed)) a made up criticism on your part.