Now the 2-11-4 MLS side looks to bounce back against the visiting Vancouver Whitecaps (8-4-6) on Wednesday at BMO Field.
A tired Toronto side, which had tied 1-1 in Dallas last Wednesday, was repeatedly carved open in Philadelphia. The Union had 62.3 per cent of possession and made 456 passes compared to just 264 for Toronto.
Mariner, who is 1-2-4 at the Toronto helm, still managed to see some positives.
"We played terribly and had 10 shots on goal," he said Tuesday. "We held our hand up after the game but we still had 10 shots on goal as an away team with seven games in 23 days. So I'm not moaning too much."
Still the loss, which ended a five-game unbeaten streak, made for poor watching for a TFC fan.
Defender Richard Eckersley called it inexcusable.
"We just lacked cohesion. It looked like we hadn't played together for a while," he said.
Added fellow centre back Logan Emory: "One of those things where everything went wrong. It's hard to pinpoint one thing."
Mariner downplayed the lopsided possession stats but Philadelphia's control of the game meant Toronto players were forced to chase the ball in searing heat.
"We had a honest discussion with the players after the game and we've been chatting with them today," Mariner said. "I just hope that they're going to be ready to go.
"I think we'll make a couple of changes but where is yet to be determined."
Toronto lost its shape and was too narrow across and up the pitch. That left the makeshift centre back pairing of Eckersley and Emory exposed and the Union profited as a result, using accurate passing to connect to Freddy Adu and Jack McInerney.
"A lot of the players have logged a lot of minutes," said Toronto midfielder Terry Dunfield, a former Whitecap who has been in good form under Mariner. "I think you need to give Philadelphia credit. They were very good on the day and they were able to pull us out of our shape, which since Paul has taken over has done so well for us."
Mariner is extremely positive with his players in public and took the time to speak to the beleaguered Emory after he was substituted Sunday. But players said there were harsh words for all in the privacy of the locker-room.
Mariner and TFC are actively looking for help, especially in the backline where defender Adrian Cann and Dicoy Williams are still struggling after long-term knee injuries. Veteran Ty Harden has an adductor strain, Chilean Miguel Aceval has fallen out of favour and teenager Doneil Henry is coming back from an allergic reaction.
That has left a back four of Jeremy Hall, Eckersley, Emory and Ashtone Morgan, whose average age is 22.
Eckersley, a fullback who spent one year as centre back with the Manchester United reserves, has been pressed into action at the centre of the defence because of the injury issues.
The preferred midfield has featured captain Torsten Frings and Dunfield in the middle with Eric Avila and Reggie Lambe on the wings. Danny Koevermans and Ryan Johnson play up top.
Mariner is short on options outside his match squad. Young winger Joao Plata has been returned to his Ecuador club side in a messy loan deal and the team is on the lookout for reinforcements.
"We're close but I'm loathe to say that we're inches away. We're not," Mariner said. "I've been at this game too long. Until people are signed, it's miles away."
The Whitecaps, on the fourth leg of a five-game road trip, have gone 3-1-3 since losing 1-0 in Toronto in the second leg of the Amway Canadian Championship final on May 23.
"They're a difficult team to beat and that's the one thing that we need to be," said Mariner.
Toronto, which had yet to win in the league prior to that Cup game, has gone 2-2-4.
After a day off Monday, Mariner went easy on his players Tuesday. The starters had a short gym and recovery session while the rest of the squad practised in the heat outdoors.
Eckersley expects Vancouver will come to BMO Field with a point to prove "because every time they come to Toronto we obviously get good results against them.
"But we've come off a bad result so we're trying to prove something as well."
NOTES — Toronto has Bermuda international goalie Freddy Hall in on trial. With Stefan Frei sidelined with a long-term leg injury, Mariner only has 17-year-old Quillan Roberts to back up starter Milos Kocic ... Designated player Julian de Guzman, whose current deal with Toronto expires in December, says his representative and the club are in the early stages of contract discussions. De Guzman says his hope is to stay in Toronto, adding "we'll just have to wait and see how both sides feel about it." ... Vancouver will be without suspended striker Eric Hassli.