The decision has been lambasted in both Spain and England.
But Clement argued in the Daily Mail: "There is a lot of competition for places here. Gareth came and there were no guarantees of anyone playing regularly and Mesut wanted that guarantee.
"It couldn’t be given and so he decided to move on. He has got the experience of playing four years here at a very big club and he goes into a club that has probably underachieved now for a good number of years. He goes into a dressing room where he is a senior figure — it’s a perfect time in his career for that."