Ivan Rakitic has warned Barcelona's Champions League rivals that the Spanish champions are "not at 100%" despite an impressive 4-0 win over PSV Eindhoven on Tuesday night.

Lionel Messi scored a hat-trick while Ousmane Dembele netted the other as Barcelona opened their 2018-19 Champions League challenge with a four-goal success over PSV.

Rakitic, however, has said that his team still have a lot of improving to do in the coming months.

"It's not easy, this is the Champions League and also our first match against a team who have started well in their domestic league, but it's been a game that worked out for us," Rakitic told reporters.

"We did many things that could be enjoyed, but we have things to improve on and we have to keep working. You can always improve, but we've just started out. We talked before the game and prepared for it, but we're not at 100% yet so we'll keep working."

The Catalan giants won the La Liga title and Copa del Rey last term but were forced to watch bitter rivals Real Madrid land their third straight Champions League crown.