Danny Racchi did grab a late goal for Kilmarnock. But Shiels says they could have snatched a draw at Celtic Park as he believes they were denied a late penalty when Fraser Forster challenged Paul Heffernan in the box.
And the Killie boss wants a positive reaction tonight at McDiarmid Park.
Shiels said: “Against Celtic in the first half we had invention and craft and half-chances. They got in behind us once in stoppage time and had a cross deflected over Mo Sissoko for Samaras to score.
“That changed the game and they scored again after 53 minutes when Sissoko slipped and Samaras took advantage.
“For 20 minutes after that there wasn’t much going on. I said to the players there was a resignation about them.
“It was like: ‘Phew – the game’s over’. It shouldn’t have been like that. I gave them a toasting afterwards and told them you don’t give up in football.
“I’m not saying they deliberately gave up but subconsciously they did, which hurt me. We need to deliver in these situations.
“We had a chance to come back and get a point. And we might have if referee George Salmond had given what was a stonewall penalty in the last minute when Fraser Forster took out Paul Heffernan.”
But Killie midfielder Rory McKeown denied the Ayrshire side had downed tools and said: “We felt we played well at Celtic and the result could have gone either way. The gaffer sees the match in the way he sees fit but the players were trying their hardest on the pitch.
“Once the second goal went in it hurt us. We were thinking this could be four or five but we were also thinking if we get a goal then we were right back in it.
“Kilmarnock have taken points off both members of the Old Firm and we have beaten Rangers.
“St Johnstone are a big test for us tonight. But this is a game we will go into thinking we can win.”