Goals from Ryan Cresswell and Britt Assombalonga put the Shrimpers in firm control, but it was only in the immediate aftermath of Assombalonga's second that Still managed to make his tactical changes.
Sam Williams, Dominic Green and Dwight Gayle came on and duly helped the Daggers get on top in the closing stages, all causing problems in the final third. But it was already too late with Southend having opened up a two-goal advantage.
"It was frustrating that we didn't manage to make the subs until we were already two goals down as they changed the game," Still said. "The difference was massive up front and we looked dangerous creating chances and pressing the Southend defence, but it was too late as the damage was already done.
"I expected a little bit more from one or two people who came in because it was an opportunity for them to show how close they are to the starting 11 but they were nowhere near close enough.
"I had to give those players the opportunity out there because if anyone gets injured they are the people who will be filling the spaces. But can they fill the spaces? No, not after that display."