For the second time in the space of four days, Latics had to dig deep in the closing stages to rescue a point from a game they had earlier threatened to dominate.
Whereas Leon Barnett had been Tuesday night's hero against Doncaster, this time it was substitute Jordi Gomez who stepped up with just five minutes remaining to slot home a free-kick and make it 2-2.
The hosts had led through Grant Holt's early penalty, but found themselves trailing when George Friend equalised before half-time and then Grant Leadbitter put the Teessiders ahead inside the final 15 minutes.
Gomez, though, bagged his first goal of the campaign with five minutes left after Roger Espinoza had been fouled on the edge of the penalty area.
"We lost two more avoidable goals, and that is proving to be a real problem at the moment," Coyle said.
"The first goal has taken a deflection off Emmerson Boyce, and it should have been dealt with in the wide area before it came in.
"The second goal was just a hopeful punt up the field, which we've allowed to bounce, and we had two opportunities to clear but failed to.
"We're not shutting the door at the back, and ultimately you're paying a heavy price for that.
"Having said that the character was there in abundance and we could easily have won the game.
"We could easily have had three penalties in the first half that would have seen us out of sight."
Coyle is expected to bring in another striker before next week's transfer deadline, a search that could intensify with Holt limping off in the second half.
"There's no secret we're looking to add another striker to the group," Coyle added.
"We'll certainly try everything we can before the window closes and, if we can't do a deal, then we'll look at the loan market."