The midfielder has been one of the great success stories across Europe this season after leaving Southampton last summer.
"In England they were really looking at him. Tadic, is he that good? But of course he also plays in a different role now. With Southampton he played on the outside; now he plays in the centre," Gullit told Rondo.
Former Schalke coach Youri Mulder also stated: "He played the World Cup with Serbia. He then had one week of vacation and a few days to prepare for the match against Sturm Graz. From that moment until now he has been there."