In his opinion. From the BBC:
http://www.bbc.com/sport/football/43122138
"I thought it was a reckless challenge, and rightly a caution. I thought, 'you've got this spot on'. You can clearly see the yellow card out, and the referee writing Fabian Delph's name and number on the card, and getting ready to issue it. To my amazement, he then puts the yellow card away and reaches for the red. At that level, don't do that. If you're not sure, don't give a card, give yourself some thinking time. For me, once you've got it out, rightly or wrongly, you've got to go with your decision. He's on the elite group of Uefa referees. If he did that in the Champions League, I don't think he'd referee again in it this season. For a referee of his experience to do that... it just shouldn't happen at that level. It's different if he hadn't pulled the card out, if he's getting information from his colleagues. Once he's got that yellow card out he's got to go with that, he can't then put it away after writing he name down. It's just basic refereeing."