Having taken a first half lead through a Marko Arnautovic penalty, the Hammers had every reason to be encouraged at the break, but things reverted very much to the nervy, erratic West Ham of last season after the restart.
Goals from Calum Wilson, who was given a free run by four claret shirts to equalise, and Steve Cook turned the game in favour of Eddie Howe’s side.
And, when asked whether or not it was experiences from last season causing the problems, Pellegrini said: “I think so, yes...yes.
“That is one of the reasons why in the second half we changed so much trying to keep it 1-0.
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“What they need is to try to work the 90 minutes in the way they did [the first] 45 today.”
He continued: “We knew before that we need time when you have so many new players, a new manager and style of play.
“But, most of all, we need to forget about last season. We need to recover the trust in each player and what they can do, we need to recover the trust as a club and that is what me must do this week.”
A trip across London to Arsenal comes next, where the Hammers will have to be far more structured and confident if they are to prevent starting another season with three defeats on the bounce, as they did a year ago.