The manager must make a decision on whether to call back Harewood from Wolves tomorrow, before Villa's clash with his former club West Ham United.
Agbonlahor underwent a scan on a hamstring problem he suffered in last weekend's 3-3 Premier League draw with Everton at Villa Park.
"Gabby felt his hamstring in the Everton game and we did not want to take any risks," O'Neill told the Birmingham Mail.
"The results from the scan have come back and it's more of a strain.
"I'm hopeful that we will see him before the end of the season. Gabby is a quick healer."