Newcastle United head coach Steve McClaren wants striker Aleksandar Mitrovic to show more "emotional control" after a second booking in as many games in the 2-0 loss at Swansea.
Substitute Mitrovic, 20, was cautioned for a tackle on Swans full-back Neil Taylor moments after coming on.
The Serb was also booked seconds into his debut against Southampton.
"There was nothing in it, but there was a reaction from players and the crowd," McClaren told BBC Newcastle.
"He's a physical player and we can't take that away from him. One thing we learned from Saturday, you've got to learn about emotional control.
"Referees need to be more aware - not be lenient, but allow the game to go on."
Mitrovic, who joined the Magpies from Anderlecht for £13m in July, had been shown a yellow card in almost identical circumstances against Southampton in Newcastle's opening Premier League game.
Meanwhile, Netherlands full-back Daryl Janmaat's dismissal at the Liberty Stadium was a blow to the Magpies' hopes, with the visitors already a goal down.
That was further compounded by the loss of France midfielder Moussa Sissoko to a groin injury.
"Credit to the players - they kept going and kept their heads up, but the second goal killed us," McClaren said.
"Once we settled down we continued to play football, we were opening them up, had one or two opportunities and we felt going in at half-time we still had an opportunity.
"But it's difficult when you're down to 10."