Watkins left for Aston Villa, while Benrahma joined West Ham.
Frank said: “For various reasons, there are only 10 clubs in the world that are not selling clubs.
“So we have a clear strategy and we are so aligned through the club that, yes, we want to do the best we can and improve a little bit and make the club better and maybe one day reach the dreams.
“But on that journey we know we will develop some players and sell some players.
“We are so clear in terms of adjusting and meeting expectations of the players we are signing, they know it's two to three years at least that they are here before they can move on.
“But if we've developed them and they have done well and they have a chance to go to the next level then we are in a way — I will not say happy, because you always want to keep your best players — but we totally understand how the world works. It's remarkable that this year particularly — and I know it wasn't the right order, necessarily — but both Ollie and Said were record transfer sales from the Championship going into another league.
“It says a lot about our recruitment and how we develop players and make them better, and we are very flexible in that.
“So of course there's a sense of pride."