Dundee United manager Peter Houston will seek "clarification" from the Scottish Football Association's head of referee development, John Fleming, after claiming that referee Iain Brines told him that "on another day" he would have booked Celtic midfielder Beram Kayal for a foul on Craig Conway.
In the first half of Celtic's 4-1 win at Parkhead, Brines had a word with Kayal after awarding a free-kick for his tackle on the Tannadice midfielder as he raced down the left.
Houston was raging that a similar challenge by Jon Daly on Charlie Mulgrew in the closing stages of the game led to a yellow card for the United substitute.
Unprompted at the post-match media conference, the United boss said: "The one thing I want to mention is that I'm disappointed with the referee. I don't want players sent off or yellow-carded, it's not my style but to me, Kayal taking Conway out was a stonewall yellow card. Seconds left in the game and Jon Daly, in my opinion, does the exact same thing to Charlie Mulgrew, catches him a second too late.
"I hold my hands up, we were beaten fair and square but what annoys me is that I asked the referee for an explanation in the tunnel and his explanation was that on another day the Conway one would have been a booking.
"What's that about? What does that mean? It baffles me. I need to speak to John Fleming because it's bugging me. I want clarification on what days you get a yellow card for that and which days you don't. Simple as that."