0150 GMT September 26, 2017
With both Real Madrid and Atletico having dropped points with draws earlier in the day, Barcelona was looking to strike an early blow at the top of the La Liga table, Sky Sports reported.
Messi set Barca on its way with the opening goal after 26 minutes, although he looked to have been offside when collecting a pass from Ivan Rakitic before sidestepping his marker and lashing the ball into the top corner.
The No. 10 doubled the lead shortly before halftime from close range after a ricochet in the penalty area had fallen into the path of Jordi Alba, and completed his treble just after the hour.
Gerard Pique's header made it 4-0 with three minutes left before £96.8 million signing Ousmane Dembele, on as a late substitute, laid a fifth on a plate for Luis Suarez in stoppage time.
It was a 42nd career hat-trick for Messi, 38 of which have come for Barcelona.
"I am happy for Leo who scored three goals and is extraordinary," Valverde said at a postmatch press conference.
"We pulled through a complicated game, because these kinds of matches are always tough.
"Despite the result, it wasn't easy. Espanyol fought very well, but we have great players like Messi, who scored three goals, and Suarez, who gave everything like the rest of the team."
The comprehensive win puts Barcelona, which hosts Juventus in the Champions League on Tuesday night, two points clear at the top of the table, having yet to concede a goal in its three La Liga victories.
Despite Barcelona's positive start, Valverde wants to keep things in perspective as the Catalans look to maintain an advantage over champion Real Madrid.
"We are at the beginning of the season and the distances are not significant, but it is better to be ahead than behind," Valverde said.
"It does not mean anything, but being ahead is always good."
Valverde had resisted the temptation to throw Dembele straight into the thick of the action following the 20-year-old's big-money high-profile switch from Borussia Dortmund, but the loudest cheer of the night from the Nou Camp faithful greeted his introduction on 68 minutes, when the result was already secure.
"The idea is to give Dembele minutes, we have a lot of hopes for him and this game was a first step," the Barcelona coach said.
"His teammates know that he has real quality."