The 31-year-old is enjoying another fine individual season for the Celestes, scoring 19 goals in 34 matches across all competitions, despite the team's struggles throughout the year.
Celta are in real danger of being relegated to the Spanish Segunda, as they currently sit 18th in the Liga table with eight matches remaining this term.
The fact that Aspas has still been able to stand out under testing circumstances is a testament to the superb striker he has become and his club colleague Mendez is well aware of his qualities.
The 22-year-old has played alongside Aspas for two seasons and he has compared his fellow countryman to five-time Ballon d'Or winner Messi, who continues to set the standard as Europe's finest player.
The Blaugrana captain has already scored over 40 goals during the 2018-19 campaign and he is on course to win the Golden Shoe yet again.
However, Mendez feels that Aspas has a greater influence at Celta than Messi does at Camp Nou, as he stated: "In the end, Messi is different from the rest, Iago is also someone who makes a difference for us.
"I think they are players who, in that sense, are very similar. Their teams notice when they are not playing.
"Perhaps because of what we’re seeing this year, I could say that Iago is more important for Celta than Messi for Barca."
Mendez went on to highlight his team-mate's passion as the main force behind his success, as he added: "He has special characteristics. His courage, his desire and his passion for what he does is the biggest thing.
"I think he is a player who lives for football like few others, and who always enjoys being on the pitch."
Barcelona, meanwhile, are due to face off against Atletico Madrid at Camp Nou on Saturday , where victory would see Ernesto Valverde's men open up an 11-point lead at the top of the table.
Messi is expected to return to the starting line-up after a cameo role as a substitute during Barca's 4-4 draw at Villarreal on Tuesday, during which he scored his sixth direct free-kick of the current La Liga campaign.