The 42-year-old, now coaching Bayern Munich, spearheaded a highly successful period for the club which saw it win a wave of silverware including three Ligas and two Champions Leagues.
He left the club amid reports of bad blood with Rosell back in 2012 but the Camp Nou chief says there will always be a place in Catalunya waiting for him.
"The doors will be open for him for life," he is quoted as saying by Marca. "[He was] by far the best coach in the history of the club."
Despite his admiration for Guardiola, Rosell went on to voice his delight with how Tata Martino has fared since taking over as coach from Tito Vilanova over the summer.
"Tata is a very brave person, with a lot of knowledge of football. He has vindicated out decision to sign him."
Finally, the 49-year-old backed Lionel Messi to retain the Ballon d'Or ahead of Franck Ribery and Cristiano Ronaldo, despite an injury-hit 2013.
"If people vote for the performances last season, then, no doubt, Messi was the best. He won the last four awards and when you see the same face again and again, people will look for a change.
"It's human. But I believe it should again go to Messi."