Messi has faced criticism for a lacklustre showing in Barca's UEFA Champions League exit to La Liga leaders Atletico Madrid at the quarter-final stage last week.
The Argentina international also failed to reach his usual high standards as Barcelona crashed to a 1-0 defeat at Granada on Saturday - a result that severely hinders their hopes of retaining the Spanish title.
Barcelona sporting director Andoni Zubizarreta leapt to Messi's defence earlier this week, insisting that all is well with the 26-year-old despite his slump in form.
However, Menotti - who coached Barcelona between 1983 and 1984 - thinks that Messi is mentally burnt out, and reckons that he needs to rediscover his "joy" of playing for Barcelona.
Speaking to La Xarxa, Menotti said: "Messi has to overcome the sadness of poor performances and rediscover the joy, pride and passion it takes to play at a club like Barcelona.
"I've seen more mental fatigue than physical. It's normal when there are a lot of games and he has many obligations.
"Maybe a couple of days rest would help him recompose his head and be the best in the world again."
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