Reddington didn't miss a thing. If it happened, he knew about it. He certainty hadn't missed the looks that Agent Ressler threw at Lizzy when he thought no one was looking, and he didn't miss Lizzy's apparent need to be there for Ressler, to protect and comfort him. He noticed the pull they had on each other from day one, even if neither of them seemed to.
That was why, after another night of sitting and holding a heart broken woman in his arms, Red came to the decision that it was time the two did notice. He loved comforting Lizzy, getting to hold her close as he ran his hand through her hair to calm her, but it was killing him to see her like this, to see her in so much pain. Without her knowing his true connection to her he would never be able to be enough of a comfort to her. She needed to feel loved, be it paternal or romantic, and since he was far from ready to tell her the truth it would have to be romantic, meaning he had to find her someone. He didn't even entertain the thought of really looking because he knew automatically it had to be Donald.
After coming to the decision that it was best to fill the hole in Lizzy's heart with what she had lost, a loving partner, he started to formulate how to make that happen. He paid yet more attention to the two whenever they were working on a case, carefully observing how close they got, how often they touched, and more so in Donald's case, how often they stared. He wanted to know how far they had gotten on their own and how much he would have to push them together.
He also watched them individually; he watched for signs of how far Donald had gotten in grieving over Audrey and how closed off Lizzy had gotten since the light had been shone on Tom, he watched how they interacted with others compared to how they interacted with each other. He watched to make sure that it truly was a possibility before he put in any real effort to match them up.
It only took a few days for him to conclude that it was more than possible; that if time had permitted it, they would have most likely gotten there themselves, eventually. Time, however, was not on their side so he would have to expedite things a little. He had to do it for Lizzy, and he wanted to do it for Donald as well. He enjoyed messing with the man but he was a good person who deserved to be happy, and Red knew that Lizzy would make him happy, as would he make her happy.
It was with that in mind that Red decided what his first move to push these two together would be.