Wijnaldum now plays in the Saudi Pro League with Al-Ettifaq.
He told AD: "(Jurgen) Klopp and (Pep) Guardiola have changed football in my eyes. People like that deserve to direct their farewell.
"With Arne Slot, I think Liverpool have really found a great successor. The fact that everyone in England still doesn't know Slot fully is not a bad thing at all. I moved from PSV to Newcastle United, I had become champion.
"In Holland, I was already playing for the Dutch national team and we were playing European football, but I didn't feel like that said anything to them.
"I trained with Feyenoord for a few days at the beginning of this season and I saw how Slot worked. He was of a very high level, I thought he was really high level. I can make a comparison with Klopp and Mourinho.
" Of course, now he enters a completely different world, that's it. But that doesn't mean he can't do what he did at Feyenoord there, right?"