James Hanson, who scored both goals in the Bantams' 2-0 win over Orient, and Andrew Davies were substituted at half time at the Matchroom Stadium due to slight hamstring strains.
Influential striker Jon Stead missed the Orient match altogether after picking up a nasty cut on his ankle during the first half of Sunday's impressive F.A Cup win over Sunderland.
With City's encounter with Walsall at the Coral Windows Stadium coming only three days after their trip to London, Parkinson has acknowledged that there is no guarantee at this stage that any of the trio will be fit to face the Saddlers.
Parkinson said: "Davies and Hanson both got tight hamstrings at half time so we weren't prepared to take a risk with either one of them.
"We took them off and I have to say the lads who came on (Gary MacKenzie and Francois Zoko) did really well.
"They (Davies and Hanson) are concerns to us I suppose because they both couldn't carry on.
"We don't think they are too serious, however, because they were able to at least play on until half time.
"We knew it was always going to be a possibility that people would pick up injuries during this busy run of games we've got, so it is just a case of making sure everybody is up to speed.
"We will probably be depleted going into the Walsall game - Jon Stead will be a big doubt again for that one - so we might be patched up a bit for Saturday.
"We have said all along that other players in the squad will be getting used during this period and we'll pick the team we think is best for us come the weekend."