But the Irishman left his players with no uncertainties after slamming their heartless display in the first half in Leith.
The home side trudged off at the break with boos ringing round Easter Road after Stuart Armstrong opened the scoring for the Tangerines in the 29th minute.
It looked set to be a fifth defeat in as many games and yet more humiliation for Fenlon after a 9-0 aggregate defeat to Malmo in the Europa League and defeat to Hearts in last week's Edinburgh derby.
But, after both sides were reduced to 10 men following red cards for Kevin Thomson and Gavin Gunning issued for an off-the-ball fracas, Hibs rallied to level through Scott Robertson with nine minutes left.
Fenlon said: "I suppose that we scored and picked up a point, there is a bit of relief.
"But it wasn't a great performance to be honest with you, especially in the first half, when we were very, very poor.
"The sending-off seemed to galvanise us for some reason and we seemed to get going. I don't know why that happened. Afterwards we wanted to fight and scrap for the ball. But we didn't want to do that before.
"That was the disappointing thing. I said to the boys at half-time that they hadn't shown any passion or desire to win the game. We were beaten by our big rivals last week and we should have come into this one bouncing and with a determination to kick on.
"We showed that after the sending-off but the key is to do that before."
Fenlon reacted to United's opener by replacing Fraser Mullen with Ryan McGovern six minutes before the break.
The former Hearts youngster was booked for a heavy tackle on tricky Terrors winger Gary Mackay-Steven on nine minutes and struggled to keep up with the Scotland Under-21 cap.
"I thought that his tackle was a bit reckless," Fenlon admitted. "He's got to learn that he's up against some tricky players and that he's got to make sure to time his tackles a bit better."
Thomson and Gunning were dismissed after the Hibs man reacted angrily to the United defender's aerial challenge just outside the away side's box.
A 15-man brawl followed and the pair had to be separated again even after they were both dismissed.
But Fenlon said: "I don't know if it went head-to-head or if anybody threw anything but I think it was just a case that the two of them raised their hands up.
"I know they say you can't do that anymore but it's a game for men. If you are going to send people off like that then you are going to have a lot of people walking this season."
United boss Jackie McNamara admitted the result was two points dropped for his side but hinted he may appeal Gunning's part in the red-card drama if television footage supports the player's claim of innocence.
He said: "I have spoken to Gav and he said he didn't throw a punch. The referee said that they were fighting, so you can take what you can from that.
"I think the two of them adopted an aggressive manner, which merits a yellow card. It changed the game and gave their crowd something to cheer about.
"We lost Gavin then had to put Paul Paton back to right-back when he had been excellent for us in midfield and we lost our way a bit. But it didn't really punish Hibs.
"I'll consider appealing it. We'll look at [the incident] again, I've not had a chance yet. But we appealed one last year that I thought was a stonewaller [that we lost]. It's £1,000 a time to appeal. It's a lot of money."