For the second time in the space of four days, the 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.
Wigan 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."
Boro boss Tony Mowbray admitted he had left a "disappointed dressing room" after seeing two points slip through their fingers.
He said: "From my point of view, we have a disappointed dressing room, because we thought we had done enough to win the game.
"We worked their goalkeeper a lot more than they worked ours, and we are very disappointed we didn't win.
"We restricted them to a penalty that we gave them and a free-kick that my goalkeeper says is going straight to him and then takes a flick off someone on the way in.
"I suppose that was a by-product of the way we play. We got caught in possession and had to give away a foul on the edge of the box.
"It is a little bit deflated in there, but there are plenty of positives for us going forward.
"This was a difficult game but Wigan are a top side, who played in the Premier League last year, and who spent a lot of money in the summer."
- Sports & Recreation
- Jordi Gomez
- Leon Barnett