[Southampton FC](http://dailyecho.co.uk/southamptonfc?ref=au) Women had held on for over an hour against the Hammers before quality options from the bench made the difference in London.
Saints, who have suffered a swathe of injuries already this season, were weakened by substitutions as they went on to concede five times in 30 minutes.
A brace apiece from France international striker Viviane Asseyi and German forward Shekiera Martinez, along with a Ffion Morgan finish, completed the scoring in a game that got away from Saints in the closing stages.
The defeat leaves Saints with three points and a minus seven goal difference after three group stage games ahead of their final tie against Charlton on Sunday, November 23.
Parker wants his side to focus on a positive first half performance but insisted they cannot capitulate when the going gets tough.
"I was really happy with the first half. I thought we competed and we were really decent in the press and we were trying to play football. It was really competitive," he explained.
Simon Parker's Saints beat Portsmouth twice in one week. _(Image: Southampton FC/Stuart Martin)_
"We knew we were going to modify some players in the second half. We took some off, which made a change to the game. We had younger players with less experience.
"But it was still a good side. They brought on a few more that strengthened them. The goals that we conceded... we conceded from a corner and from a throw-in.
"Then we got caught from a clearance and two balls over the top. Ellie Hack got caught under the floodlights but we need to be better in the second half.
"West Ham were really clinical with the chances they had. They didn't have a crazy number of chances, but when they did have chances, they were ruthless.
"We can't capitulate like that when adversity is against us. That's something for us to really work on. Then we got disjointed. Players played individually rather than as a collective.
"I was comfortable with the team coming into the second half, because we had competed with a WSL side, so there are positives to focus on."
He added: "They were more clever in the duels, the way they lean into you, pull players, they are just really intelligent in those moments.
"So when we start losing those duels, when we want to be aggressive, you put yourself into a very vulnerable situation. The experience shone through. This was a big learning curve."
Saints, fifth in the league, have now lost four straight games across the WSL 2 and League Cup, denting their hopes of earning one of three promotion spots.