A single goal from home debutant Freddie Ladapo towards the end of the first half proved enough to secure all three points for Shrewsbury, handing City just their fourth league defeat of the season in the process.
Following Ladapo's opener for the Shrews shortly before the interval, City tried to mount a response but, in truth, McCall's men weren't quite able to do enough to put Shrewsbury's resolute defence under the required pressure.
McCall said: "There was nothing tactical or magical that made us lose the game today (Saturday), we just had too many players under-perform.
"That is something we haven't been able to say too many times this season after twenty-seven league games so far.
"We've had a lot of draws obviously this season but not many of them have come about due to poor performances.
"It is very rare we have come away from a game thinking 'we were poor this afternoon' but today was one of them.
"On our day, we can be one of the top teams in this division but a performance like this one makes us look ineffective.
"You could say that three or four of the lads had their poorest performances of the season.
"We are at the stage of the season now where we can't look and say 'we've had a good start, or we're doing well' - we have to focus what we're going to do in these remaining nineteen league games.
"We certainly can't afford many more days like this.
"This defeat wasn't down to a lack of desire or effort but we were sloppy defensively and we seemed to lack some creativity in the final third.
"Our creative players were off their game today, there is no doubt about that.
"The first goal was also always going to be crucial, especially away to a side who are fighting for their lives.
"We know about their strong home record, certainly defensively, in recent months and, credit to them, they have got their blocks in and made their tackles when needed in the second half.
"The pitches are going to start to deteriorate so we know we are going to have to look at playing differently at times between now and the end of the season.
"We did look average today - the lads know that themselves - but I have faith, confidence and belief in this group that we will bounce back in the right manner."
McCall says City will check over the fitness of James Meredith after the Australian left back picked up a knock late on the game.
Meredith appeared to come off worse following a tackle with Shrews midfielder Jim O'Brien as the game approached injury time, with the defender replaced by Rory McArdle shortly afterwards.
The City chief added: "He (James Meredith) took a whack and he was limping a bit so there was no point keeping him on.
"We threw Rory on to give us something else in their box late on.
"Mez will have ice on his ankle after the game and it is something we will look at over the next 24 hours.
"Hopefully it is nothing too serious."