Liverpool finally found some rhythm and David Ngog had a chance to equalise but headed straight at Andreas Isaksson.
They have already drafted in Philipp Degen, fellow full-back Andrea Dossena, goalkeeper Diego Cavalieri and teenagers David Ngog and Emmanuel Mendy.
The midfielder cajoled his stuttering side into a greater urgency and, after David Ngog and Raul Meireles missed two gilt-edged chances, it was left to the Reds captain to grab an equaliser.
Birmingham hit back as Liam Ridgewell bundled in James McFadden's cross and survived three late David Ngog chances.
Benitez introduced David Ngog and summer-signing Alberto Aquilani in the latter stages but Arsenal close out the latter stages in relative comfort.
Houllier admitted that conceding two goals in the opening 16 minutes at Anfield, with one each from makeshift strike-force David Ngog and Ryan Babel, made things very tough for his young side.
Liverpool had Joe Cole sent off on the stroke of half-time but later took the lead through David Ngog and looked set to hold on for the three points until the visitors were handed a late leveller by Reina.
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David Ngog swivelled and had a shot blocked by Napoli's Salvatore Aronica as both teams had to settle for a draw.
But, there were no signs that Liverpool were feeling the effects of their exertions in the week as they started brightly with chances falling to David Ngog, Maxi Rodriguez and Aquilani.
"You have Maxi Rodriguez, David Ngog, Ryan Babel, Joe Cole - they are quality players and it is a very strong squad and they look organised, " he said.
Liverpool, who were without the injured Steven Gerrard and Fernando Torres, showed their defensive discipline to keep out Spurs and should have been out of sight by the time they were awarded an injury-time penalty after David Ngog was brought down by Sebastien Bassong.
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