"We were below par. It's as simple as that. And it's a reality check of where we are in our one-day cricket at the moment," Arthur was quoted as saying by www.cricinfo.com on Sunday.
Pakistan grassed an array of catches and were also guilty of taking some unwise decisions on the field. Arthur said they erred on the "simple things".
"The worrying thing for me -- and it has been for a period of time -- is we just do the basics wrong. We do the simple things wrong. We drop simple catches. We don't run well enough between wickets," he said.
"We don't hit the keeper with our throws and we don't understand when to bowl our variations. We bowled a really good over and then we bowled a variation with our sixth ball and that's the ball that ended up going for a boundary. So, it's the simplicity of those things that is worrying me at the moment," he added.
Arthur opined the magnitude of the occasion got on to the side relatively thin on international experience.
"We talk about setting the tone up front. But we didn't set tone up front. We had a couple of guys that were in the clouds at the start and that sets tone. For us to have a performance as tentative as that right from the start is a worry and the only thing I can think of is the magnitude of the occasion got to them," Arthur said.
"My issue is fear. My issue is them getting out there and really looking to take the game on and just believing in themselves and believing that they can take the game on," he added.
Pakistan were further troubled by fitness issues as pacers Mohammad Amir and Wahab Riaz sustained cramps on the heavy underfoot conditions in Birminghamm and had to leave the field without being able to complete their overs. Hasan Ali also suffered an attack of cramp.
"I don't know why they're cramping. That's something that I need to take up with the medical team," Arthur said.
"Wahab was fully fit. He went through a fitness test. He was declared fully fit. He performed poorly today. But he had a role to play. And I'll take the blame, me, I'll take it. I selected him. I selected him because I wanted him to perform a role. He didn't execute that role, unfortunately."
"So sometimes it works out. Sometimes it doesn't. We picked Shadab Khan ahead of Fahim Ashraf today. That worked really well. I thought Shadab was outstanding. So, sometimes it works. Sometimes it doesn't," he added.
Arthur though backed his side saying it's unfair to judge their mettle on the basis of one bad day.
"It's a total insult to say we're playing even worse," he said.
"If you have a look at our records over the last year, we've won two series. We've got ourselves from No. 9 to No. 8 in the rankings and our brand of cricket has changed," the former South Africa and Australia coach said.
"We had a poor game today. But we're obviously trying our best and we're trying to change it. It's not going to happen overnight. We're trying every day. Every time we go down to training, we try and get the basics right. We didn't do it today. And that's disappointing."
"We'll have a good, hard chat tonight. And then when we leave the dressing room, it will all be done and we'll be thinking firmly of coming back and beating South Africa. Because that's what I think our players can do. So, we're going to be up for the fight."
"I'd be really disappointed if we walked away and for the rest of the tournament sort of hit our heads because we're better than that," he added.
Source - Agency