The Spain striker has been linked with a move away from Sevilla, with president Jose Maria Del Nido warning players may have to be sold to make up for a deficit in the cash-strapped club accounts.
But the 27-year-old proved his worth by coming off bench in the 65th minute, having missed the previous three games with a hamstring problem, and converting a Coke cross to silence the Estadi Cornella-El Prat.
The draw represented a huge opportunity missed for the Catalan side, who led twice through Joan Verdu's early penalty and Simao's header after an hour and were a man up from the 39th minute following the dismissal of Jose Antonio Reyes.
Ivan Rakitic's penalty midway through the first half made it 1-1 at the break, but it was Negredo's last-gasp leveller which really hurt the hosts, denying them the chance to move out of the bottom three.
Espanyol had their goalkeeper Cristian Alvarez to thank for preventing them falling behind after only five minutes as he produced a fine parry to keep out Emir Spahic's angled shot from the right side of the area.
And the hosts went in front after 13 minutes when Fernando Navarro fouled Verdu in the area and the midfielder took full advantage by coolly placing the penalty low down the middle.
The lead lasted just 11 minutes, though, as Espanyol handed their opponents a penalty of their own.
Christian Stuani was adjudged to have fouled Baba Diawara at a free-kick and Rakitic sent the resulting spot-kick into the bottom-right corner.
But, just when it looked as though the momentum had swung in the visitors' favour, they shot themselves in the foot.
Reyes was sent off after being shown his second yellow card in the space of seven minutes.
Espanyol appeared to sense this was an opportunity they could not afford to pass up and Verdu volleyed weakly wide from inside the area soon after the restart.
But they went back in front on the hour mark as 5ft 7in Simao headed an inswinging Wakaso Mubarak corner from the right low into the net.
At the other end Alvarez had to save with his feet to keep out a fierce Rakitic effort.
But, just as it looked as though they would hold on for victory, Coke crossed from the right and Negredo - on for Diawara - powered a header high into the net.