Doctors stablised Bella and an ambulance came onto the pitch to take him to a nearby hospital.
"He's under observation," Velez coach Ricardo Gareca said after Newell's snatched a one-point lead in Argentina's "Final" championship.
The match was a dress rehearsal of sorts for the teams' first leg of their South American Copa Libertadores last-16 tie at Newell's in Rosario on Wednesday and both rested several first choice players.
Striker Martin Tonso put Newell's ahead in the 19th minute when he turned two defenders and curled a left-footed shot inside the far post.
Velez, winners of the "Inicial" championship in the first half of the season, equalised through Ezequiel Rescaldani in the 56th minute.
Newell's regained the lead seven minutes later with a shot from outside the box by Paraguayan defender Marcos Caceres.
Substitute striker Maximiliano Urruti made sure with a superb shot from the right edge of the box into the top far corner in the 77th minute.
Newell's have 22 points, one more than Lanus who can reclaim the lead when they play at Union in Santa Fe on Monday. River Plate, away to Godoy Cruz on Sunday, are third with 17 points.
Velez host Newell's again at El Fortin in Buenos Aires in the second leg of their Libertadores tie on May 14.