"We'll make changes after that performance, but there's key areas in the team that we need to strengthen."
Hibs conceded inside the first three minutes as Johnny Russell fired home and second-half goals by Jon Daly and Michael Gardyne completed United's victory.
"You spend days preparing your team and organising and you concede a goal after two or three minutes," said Fenlon.
"It gives you a mountain to climb.
"I'm disappointed that they didn't carry out what I asked them to carry out. That's the major disappointment.
"I'm angry. I'm disappointed more than angry. The frustration of not being on the bench doesn't help as well."
Fenlon has added goalkeeper Ben Williams, midfielder Paul Cairney and defender Tim Clancy to his squad from last season, while striker Leigh Griffiths and captain James McPake have returned to Easter Road after time on loan there last season.
Latest signing Gary Deegan, the midfielder secured from Coventry City, was not registered in time to face United.
"We've released a lot players since the end of last season and we need to bring players in," added Fenlon. "We're a little bit short.
"If you look at the striker options we had on the bench. We were able to bring on two kids and that's really it.
"You need competition for places within your squad as well."
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