The Dutchman has come under fire at Goodison Park recently after a series of weak performances, questionable team selections and poor results.
Koeman handed the 31-year-old his senior debut at Ajax over a decade ago and the pair came close to being reunited this summer.
"I don't want to go into details," he said, when probed about suggestions Everton had attempted to sign him on deadline day.
"Barcelona always said they didn't want to let me go. All big clubs need at least four centre-backs.
"They were happy with my pre-season and kept me."