The Hammers have already added eight new players to the squad following Pellegrini's appointment as manager, smashing their transfer record to sign Felipe Anderson from Lazio on Sunday.
Having completed a deal for top transfer target Anderson, it was expected the east Londoners' business would begin to tail off but Pellegrini says that may not be the case.
Carroll and Reid have both been ruled out for up to three months after aggravating old injuries training at the club's pre-season camp in Switzerland, forcing West Ham back into the transfer market.
"We are obviously very sad to hear that Andy and Winston have suffered setbacks," the Chilean told Football London. "This is a big blow for them and us, but we hope to see them back in action as soon as possible.
"It does mean that we have lost two senior players in key areas, though, and we'll continue to work hard with the owners to make further additions to strengthen the playing squad beyond the five new signings already confirmed."