He and fellow substitute Steven Naismith reinvigorated their team as second-half substitutes in Sunday’s 3-2 defeat.
However, after an eye injury suffered against Stenhousemuir last month, Washington had to convince manager Craig Levein he was ready for the Premiership's opening weekend.
“I thought about throwing them both in from the start but it was two different situations,” Levein told the Evening News. “Washington got a bang on the eye and there were conflicting opinions among the medical people on whether he should be involved or not.
“The only reason he was involved was he came to see me and said: ‘I want to at least be on the bench and see if I can get on.’ That’s why I put him on.
"I had a chat with Naisy the other day. He had only played 60-odd minutes since he got injured way back in February. I know he’s an experienced player but if he starts the match and I have to take him off – which I would have done because he has hardly played – that’s a negative. I wouldn’t be putting anybody better than him on.
“I didn’t know how long he would last. I knew he could definitely do half an hour if we needed him.”
Naismith levelled the score at 1-1 with his first touch and Washington’s energy troubled the Aberdeen defence. That helped Hearts take a 2-1 lead before they were reduced to ten men and eventually lost out 3-2.
“The idea was to put Naisy on the teamsheet, let the opposition see he’s there, and let them see Washington was there as well if we needed them. It got to the point we did need them.”