Mascherano arrived at Camp Nou in 2010 from Liverpool regarded as one of the best defensive midfielders in Europe, but he has been transformed into a top class centre-half under Guardiola.
The former West Ham United man had another excellent game in the 2-0 victory over Athletic and his boss admitted afterwards that he considers the 27-year-old one of the most valuable members of his star-studded squad.
“Javier is an almost unimaginable signing," the Blaugrana trainer is quoted as saying by his club's official website.
"He’s the type of player that a manager wouldn’t trade for anything, and would never sell. He’s a unique signing, we’re very pleased to have him."
However, Guardiola acknowledged that Mascherano was not the only impressive performer against the Basques.
“We’ve missed Gerard for the best part of the year but now he’s back to his required playing level," the former Spain international argued.
"He and Mascherano played a great game. And I also want to highlight Adriano’s effort.”
The win over Athletic sets Barca up nicely for the second leg of their Champions League quarter-final tie with AC Milan but Guardiola is warning fans not to expect a similarly open game against the Italians, with the two sides having drawn 0-0 at San Siro on Wednesday night.
“Athletic Bilbao don't defend - they come for you," he explained. "It’s completely different from Milan; it’s like night and day.
"However, while it will be the most important match of the year, it could also be the most beautiful one.”
Barca's meeting with Milan at Camp Nou gets underway on Tuesday at 20.45CET.