Cody Gakpo opened the scoring with a header from Robertson’s cross in the 20th minute, followed by Luis Diaz doubling the lead just 90 seconds later with a close-range finish.
Despite leading at half-time, the Gunners leveled the game with goals from Gabriel Martinelli and Mikel Merino.
"I thought in the first five to 10 minutes both teams were sloppy," Robertson told Sky Sports.
"I think we picked up, scored two really good goals and probably had a couple more chances.
"In the first half we dominated. At half-time we went in and said to keep the same intensity and all the usual stuff and we come out in the second half in the first five to 10 minutes and we were miles off it.
"I think they tweaked a couple of things that started causing us a bit of problems. But we probably lost our aggression in the press and making our last step. Then they slowly but surely come back in the game.
"I think we finished strong. Obviously there's a man sent off and a couple of chances. I thought I nicked it at the end but it wasn't meant to be. I was told I was just offside anyway so the foul is probably irrelevant.”