The 22-year-old was on the bench but remained an unused substitute, with James Scott the only change as he replaced Scott Allan at half-time.
Gullan then went on to score a hat-trick as the Easter Road side’s B team defeated Huddersfield Town 4-2 in a friendly match on Tuesday and Ross held his hands up over his decision not to bring ‘Hammer' off the bench.
"I told Jamie this – I made a mistake in not involving him in the game last week at some point,” Ross said ahead of Saturday’s trip to Aberdeen.
"He knows he’s got a relationship with me where I can be quite open and frank with him when maybe I didn’t get things right.
"Over the last few weeks he’s probably been as good as I’ve seen him for a long time here. It’s not just his performance on Tuesday – that would be a knee-jerk reaction – because he is consistently performing well now.
"He’s given me much more to think about but doing what he did during the week just adds to that.”
Ross praised Gullan for his efforts in training as he confirmed that the attacker had worked his way back into the first-team plans.
“He’s put himself very much back in our plans, not just by his midweek performance, but how he's been doing in general, which is great for him and good for me given what I’ve seen from him on the training pitch every week.”