Arne Engels, Celtic's record signing, was valued between £14m and £19m after his move from Augsburg.
After his Champions League and domestic performances, his value soared to £17.75m-£25.3m.
Now, CIES have valued Engels between €15m-€17m (£13.05m-£14.79m) after his first full season at Celtic.
This value compares to Rangers' Nico Raskin, another Belgian midfielder.
CIES have valued Engels between €15m-€17m (£13.05m-£14.79m)
Author's summary: Engels' value remains high despite struggles.