The Welshman insists the Blaugrana's difficult spell on the pitch is as a result of their off-the-field problems coming home to roost this season
Barcelona have endured a turbulent spell under coach Gerardo Martino, losing three consecutive games in all competitions.
The Catalan giants were eliminated by Atletico Madrid in the quarter-finals of the Champions League and their La Liga title hopes were dealt a major blow after suffering a shock defeat to Granada to slip seven points adrift of Diego Simeone's men in the race for top honours.
And their woes were compounded midweek after Real Madrid won late on in the final of the Copa del Rey.
Talisman Messi failed to find the back of the net during that period but Toshack, who took charge of Real in two separate spells, says criticism of the Argentina international is unwarranted and believes the off-field problems at the club are affecting the team.
"It's cruel to blame Messi. You can't put the blame on him. Messi's at a high level but more is always expected from him," Toshack told La Xarxa.
"Neymar is a player at (Gareth) Bale's level but all the problems off the field haven't helped anything.
"Everyone was talking about Barca being the best team in history and they were one of the best of all time, but you have to renew before it's too late.
"Fix the roof before the water comes in. Don't expect to be able to fix the roof when the water's already coming in.
"Everything that has happened at Barca is taking its toll."
The future of Martino has been cast into doubt following the club's poor run of results, with reports linking him with a return to Argentina at the end of the season.
But Toshack believes injuries to a number of key personnel have had a damaging impact on the team's fortunes.
"Injuries to Victor Valdes, Gerard Pique and Carles Puyol, Tata Martino has had no luck," the 65-year-old said.
"I don't like Javier Mascherano as a centre-back. It's a very important area of the pitch.
"Barca have a job on this summer. They have to sign a centre-back."