Alex Revell gave Rotherham the lead after 31 minutes, but Evans' side paid the price for a host of missed chances and Adebayo Akinfenwa salvaged a point for the visitors with an injury-time equaliser.
Evans said: "Northampton didn't have a shot on goal and were outplayed, but it was disappointing to concede like that right at the end.
"But we know what we lack and that's a bit of quality in certain areas, which is what we identified when we came here.
"It's an immensely proud day for me because my Crawley team has been promoted. A number of players have been on the phone this week and I've been on to the owners.
"I'd like to send my congratulations to them because that's the standards we are trying to attain next year. I'm proud of what we've done at Crawley Town and now we have to do it here. We can't wait for next season and starting at a fantastic new facility."