The Armenia international moved to Old Trafford last summer
after three years with Dortmund, during which he scored 41 times and provided 49 assists in 140 appearances in all competitions.
“Manchester United are a bigger club than Borussia Dortmund, everyone knows that and we should just accept that.
“It was a new challenge for me that I could not turn down.”
The 28-year-old also laughed off insinuations that he left Dortmund because of money.
“I find it hard to understand criticism that I joined United for the money. That is simply wrong,” he added.
“Had it all been about money for me, I would have left for Shakhtar Donetsk for Anzhi Makhachkala rather than Borussia Dortmund.
“But that was not what I wanted. I wanted to keep on developing.