Saints have recalled Charles from his loan this week.
Rohl said: "It was very late, it was in the morning when I got all the information from the club.
"I must say that I’m really sad, but this is football. I was hoping until the end that it didn’t happen. I had a conversation with Shea.
"I highly rate him because of how he’s developed and improved. He’s a big player for us. I’ve seen some good players at a high level and what I see at his age...
"How he’s developed, his openmindedness and character, his game understanding and intelligence… I’m convinced that he will have a great career in a top division.
“Maybe there’s a small chance to get him back, but I don’t know at the moment. It’s a bigger picture. But what I can imagine is that it will be very tough to get him back.
"Let's see... I just have to say again, he’s an outstanding player who gives us something special. It will not be easy to have a solution immediately.”