Linfield manager David Healy told BBC Sport NI that he "nearly booed himself" after watching his side's "unacceptable" performance on their 2-0 defeat by Portadown at Shamrock Park on Friday night.
Second-half goals from Baris Altintop and James Teelan secured an impressive victory for Niall Currie's side and condemned the champions to a second reverse on the road during this campaign.
"Really disappointing. Our performance levels were nowhere near the expected levels," said Healy.
"That's why we lost. Our attitude was wrong, which is even more disappointing, because I thought we had prepared going into the game. But obviously not.
"I'll have a hard look at that. I've said to the players that I think we have 15 players out of contract this year and to be honest looking at tonight I'm not in a hurry to renegotiate anything with anybody at the minute until we find out where we are as a group."
The Blues boss added that the discontent shown by the Linfield supporters at the end of the game was understandable.
"I went and clapped the supporters tonight and they booed me and absolutely rightly so.
"I nearly booed myself walking over to clap them as I felt embarrassed for myself tonight and I felt embarrassed for the players for the level of football we turned in.
"Something I've driven at this football club is a mentality and an attitude. You may not always be at your best but you go and you empty the tank.
"You wear the shirt with pride and we didn't do that tonight. I'm responsible for that, I pick the team, it's my fault at the end of the day.
"I wish I could have crossed the white line tonight because I probably wouldn't have done any worse than some of the players who played."
Healy says the squad will now turn their attention to Tuesday night's County Antrim Shield quarter-final away to Cliftonville.
"I'll assess the game, we'll be in on Saturday and then we play on Tuesday, which gives us an earlier return to try and put things right.
"When you do lose a game your most important one becomes your next one so Solitude on Tuesday night, County Antrim Shield, not the most important trophy, but it becomes your most important game."