Bent was axed by Villa boss Paul Lambert with Gabriel Agbonlahor and Christian Benteke preferred up front.
But he had only been on the pitch 11 minutes when he fired home the equaliser in front of England head coach Roy Hodgson to cancel out Shane Long's opener.
Villa deserved a share of the spoils after dominating the second period.
Long looked as if he had settled the issue on returning to the ground where he suffered a knee ligament injury last season which sidelined him for six weeks before Bent's intervention.
A point maintained Albion's fine start to the campaign - 11 points from six games - but Villa will also take plenty of encouragement from the performance of their new-look side.
Benteke was only inches away from connecting with an early dipping cross from Villa striker Agbonlahor
But Albion were the first to settle and Long missed a decent opportunity to put them ahead after six minutes.
A deep centre from Baggies skipper Chris Brunt was headed back across goal by James Morrison to Long - but he sent his header wide when he should have at least hit the target.
Albion were in the ascendancy in the early stages and debutant Goran Popov appealed in vain for a penalty after his cross appeared to strike the arm of Marcus Albrighton.
Villa started to settle and Albion keeper Ben Foster finger-tipped a long-range drive from Brett Holman over the bar.
Home midfielder Fabian Delph became the first player to be yellow carded after 26 minutes for a challenge on Brunt.
Albion were forced into a change five minutes before the interval when Brunt sustained a leg injury blocking a cross from Matthew Lowton and was replaced by Graham Dorrans.
Foster made a fine save when he had to readjust to turn away a shot from Holman which took a deflection off Billy Jones.
Benteke then carved out an opening for himself and winning a touch-line battle with Gareth McAuley but could not keep his shot down after cutting into the area.
Jones became the second Albion player to hobble off on the stroke of half-time with a knee injury and he was replaced by Gabriel Tamas.
Long had the ball in the net five minutes into the second period only to have strayed a yard offside.
But the former Reading player made no mistake when breaking the deadlock 60 seconds later with his fourth goal of the season.
Morrison appeared to be just offside when he picked up the ball and ran towards the Villa boss before sending a low cross into the danger area.
Centre-back Ron Vlaar only got the slightest of touches on his attempted clearance and Long was positioned behind him to make no mistake.
Villa tried to retaliate and Joe Bennett was only just off target with a curling free-kick.
But Long should have done better when again well placed and he scuffed his shot through to Brad Guzan.
Lambert brought on Bent mid-way through the second half in place of Benteke and he quickly made an impact with an 80th-minute equaliser.
Ciaran Clark's volley from a Barry Bannan corner was blocked but Bent was perfectly placed to convert the loose ball.
Albion came close to regaining the lead when substitute Romelu Lukaku's shot deflected off Clark on to a post. The on-loan Chelsea striker also shot against the legs of Guzan before the referee blew the full-time whistle, giving both teams a well-earned point.