The Barca boss did acknowledge that not being able to train with the whole group for a few days "could be a handicap", but at the same time he admitted that "it motivates us to play against Espanyol".
After a slow opening, Barcelona took the lead after 26 minutes.
A sublime second nearly arrived in the 31st minute - Rakitic adding further to an accomplished showing by gliding towards the box and exchanging passes with Messi to draw a superb save from Pau Lopez.
Messi then doubled the hosts advantage some 10 minutes later, as he slid-in Jordi Alba's smart cut-back.
Espanyol almost pulled a goal back just before the break when Pablo Piatti rattled the upright from the edge of the box.
Dembele was finally given the nod in the 68th minute as the France global entered the field for his Barcelona debut, with Deulofeu leaving to a standing ovation following another impressive performance.
Luis Suarez could also return to action after recovering from a knee injury and featuring for Uruguay in their draw with Argentina. That could have been a lifeline for Espanyol.
Alexis starts on the bench after failing to secure transfer
Arsenal got back to winning ways against a Bournemouth side who are still yet to register a point this season. I liked his second goal today, because it's really clean finishing for any goalscorer.
Suarez's trials continued as he looped an attempt on to the roof of the net having been set up by Messi - both forwards jogged back into position wearing rueful smiles.
Espanyol: Pau Lopez 7, Sergio Sanchez 6, David Lopez 6, Hermoso 5, Aaron Caricol 5, Pablo Piatti 6, Pape Diop 6, Javi Fuego 5, Sergi Darder 5, Leo Baptistao 6, Gerard Moreno 5; Marc Navarro (for Sanchez, 68) 6, Sergio Garcia (for Piatti, 57) 6, Didac Vila (for Darder, 71) 6.
"They fought very hard, but we have players like Messi, who scored three goals, and Suarez, who gave everything, just like the rest of the team".
Pique piled further misery before Dembele's helped Suarez end his misery, sealing the match right on the end of regular time.
Lionel Messi (18) is the all-time top scorer in Catalonian derbies in La-Liga.
Espanyol has gone 17 games without a win against Barcelona in La-Liga (D3 L14), its worst run without a win in the Catalonian derby. I'm sure he's fine because he's played two games for his country.
Barcelona have two out of two wins in this new season of the La Liga.