The Royals manager told BBC Radio 5 live: "We came to Manchester United and put on a good performance as well, and we're disappointed not to be coming away with a result.
"I've got loads of positives to take from this. When you're 2-0 down at Old Trafford, that can become four or five. That showed the character of our side. We put United under the cosh, and they will be well pleased to go through."
Chelsea or Middlesbrough will face United in the final eight of the competition.