The German right-back is on loan from Cologne and hopes to tie up a permanent move following a successful season.
"At Fulham it is always possible that you might lose great players, big players, to other clubs so it is not easy," said Riether.
"It is normal that if you play well other clubs want to buy you.
"If some players do leave, other good players will come and I think Fulham will have a great team next year."
"It was not easy for us after losing very good players like Dembele and Dempsey, who scored a lot of goals last season," Riether added. "Our goal was to be in the top 10, hopefully we can reach that and then it is important to build on that for next season.
"There will be a lot of changes in the team this summer. Some new players will join, some players will leave but we will be prepared for next season.
"The Premier League is not easy but we can build step by step. Our recent form is not very good but we have played teams like Chelsea, Arsenal and Everton. Two of our three remaining games are at home so I think we can finish strongly."