The Brazilian played alongside Messi at the Camp Nou until his departure for AC Milan in 2008.
Messi scooped a record sixth Ballon d’Or earlier this month after another extraordinary year, finishing ahead of Liverpool defender Virgil van Dijk.
But Ronaldinho was reluctant to call his former team-mate the greatest player of all time.
"I can't say Messi is the best ever, but he is the best of his time," Ronaldinho told Marca.
"I'm happy for Messi because he is a friend, besides he has been a reference point for Barcelona.
"I don't like comparisons because it's hard to identify who is the best in history, there is Diego Maradona, Pele and Ronaldo."
Messi has been in terrific form once more this season, scoring 14 goals and providing eight assists in 15 appearances in all competitions.
His Ballon d’Or victory was his first since 2015, and his sixth in total after previous wins in 2012, 2011, 2010 and 2009.
It saw him pull clear of Cristiano Ronaldo, on five, with the most wins in the individual prize.
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