Messi told the Catalan giants he wanted to leave in August last year, believing he could do so on a free transfer under a clause in his contract, which is due to run until the end of the current campaign.
However, the 33-year-old Argentina international reluctantly opted to remain at the Nou Camp after Barca insisted that clause had expired and it would cost a suitor 700million euros (£601million) to prise him free and while his future remains uncertain, compatriot Mascherano is convinced his on-field contribution has not waned since.
Speaking as he was unveiled as the latest LaLiga Santander Ambassador, he said: "I see him happy in Barcelona.
"Even though the club is not doing extremely well, football-wise the way he is playing and how he moves on the pitch haven't changed from what I have seen in the past 15 years, so in that sense what I see him as extremely well."
Messi joined Barca as a 13-year-old and made a record 506th league appearance for the club in Sunday's 1-1 draw with Cadiz.
Asked what advice he would give his former international colleague as he contemplates his future, he replied: "I would never dare give any advice to Leo Messi.
"First of all, I think it's a very, very personal decision that he'll talk over within his family circle. But from the professional standpoint, it's a decision he and only he can make.
"As a friend, I will support whatever decision he makes, but I would never dare give any advice as to what he needs to do."
Former Liverpool star Mascherano, 36, made 334 appearances for Barcelona during a seven-and-a-half-year stay and boasted two Champions League successes and five LaLiga titles among his lists of honours by the time he left for Chinese side Hebei China Fortune in February 2018 before hanging up his boots after a spell back in Argentina with Estudiantes.
He admits football away from the rarefied atmosphere of the top echelons of LaLiga came as something of a culture shock.
Asked if he missed playing alongside the very best, he replied: "Yes, you do miss it and you also suffer a bit because when you grow used to having the best by your side, when you don't... Football is a group sport, so the better your team-mates are, the better you are going to play.
"After leaving Barcelona, you experience the real world of football and when we get together with former Barcelona team-mates, we do realise and we do share that Barcelona was amazing with what we were lucky enough to experience but to a certain extent, there's a huge void when you leave so in that sense, you do suffer."