Celtic suffered a 2-1 defeat to Norwegian outfit Molde at Celtic Park in the Europa League Group A last Thursday.
“The worrying thing for me in that second half last week at Celtic Park against Molde was the lack of fighting spirit,” former Celtic midfielder Hay wrote in The Glasgow Evening Times.
“There seemed to be an acceptance that the game had gone away from them and that lack of passion and commitment – with the exception of the kid, Kieran Tierney, is a concern.”
“The complete lack of intensity about the performance in that second period was as alarming as the defensive lapses – and I am being kind on describing them as lapses,” added the former Celtic manager.
Against Celtic at Celtic Park last week, Mohamed Elyounoussi took advantage of poor defending from the Hoops in the 21st minute to open the scoring for the visiting Norwegian team.
Kris Commons diverted Nir Bitton's strike into the net to level the match just five minutes later, but Daniel Berg Hestad found the target in the 37th minute to secure victory for Molde.
Bitton was sent off for a professional foul in the second half, as Celtic ended the match with 10 men.
Over the course of the 90 minutes at Celtic Park, Ronny Deila's side had 52% of the possession, took 16 shots of which eight were on target, and earned six corners.
Visitors Molde had 48% of the possession, took nine shots of which two were on target, and earned seven corners.
Celtic are now at the foot of Group A with just two points from four matches, three points behind second-place Fenerbahce.