Two minutes after the restart, Lewis Wing gave the hosts the lead but their strife for a victory was dealt a huge blow when Kalvin Phillips headed home a 102nd-minute equaliser.
“It’s a hard one to take. We’ve played really well the whole game,” Mikel told club website.
“We did exactly what we wanted to do. We were aggressive, we won the ball, and got in their faces. We tried to play and created chances.
“Everything was there today for us to have a good game.
“The fans were amazing, they were right behind us pushing us forward.
“To concede in the last minute is disappointing but we have to keep going.
“We have another big game on Wednesday and hopefully we can get the three points then.”
Next up, they play another Premier League promotion chaser in Sheffield United - placed third in the Championship log - on Wednesday.
Mikel who joined Boro on a short term deal during the January transfer window made his second appearance, playing for the entire duration against Marcelo Bielsa's second-placed side.