Brendan Rodgers was left baffled over a yellow card controversially shown to Leigh Griffiths as his Celtic side won 2-0 in the first leg of their Champions League second-round qualifying tie at Linfield.
Griffiths was showered with missiles including coins and bottles as he attempted to take a left-wing corner for the Scottish champions during the second half of the game.
After the forward protested to match referee Alejandro Hernandez, showing the official one of the bottles that had been thrown at him, he was booked by the Spaniard.
Rodgers could not understand why Griffiths was booked by the official, who was in charge when Barcelona were denied a late winner in a LaLiga game against Real Betis in January, despite the ball clearly crossing the line.
"I didn't quite see what it actually was but if that is the case [missiles being thrown] then of course there is always a worry," said Rodgers after goals from Scott Sinclair and Tom Rogic put Celtic in control ahead of the return leg at Parkhead.
"But I didn't see or haven't heard anything as of yet as to what was thrown on, I could only see it from the dug-out. It was maybe a bottle of some sort, but anything, whether it's a coin or a bottle or whatever, it shouldn't happen.
"Players go on to the field and it should be a safe environment for them. You want the passion from the stands to stay in there. But if you take that away from it, it was a good evening. You sometimes get these scenes throughout the country, not just here but other places as well.
"Obviously it's not what you wanted to see. The game was played in good spirit, in the main. So it's not ideally what you would like to see. I thought it was strange [Griffiths] getting booked for it.
"But like I say the talk is more really on the football for us. The scoreline was okay, it could have been better but it gives us the advantage for the second leg."