Zigic has been unavailable for Blues' last three games as he served out a suspension following his sending-off against Huddersfield Town.
Lovenkrands has been sidelined for the last two matches with a groin injury, which he picked up in the 1-1 draw against Leicester City - a game in which he also scored.
Consequently, Clark was not able to name any strikers on his substitutes' bench against either Millwall or Leeds.
"It's great to have options in terms of the four (which includes Leroy Lita and Marlon King)," said Clark.
"But for the two lads coming back now it's going to be difficult to dislodge the two in there at the moment because they are playing very well together.
"If players are playing well, it's up to other players to get the shirt off them."
Clark says the pair are itching to get back into the action: "They are both chomping at the bit to get on the pitch and get playing again," he said.
"To be fair to Peter he has only missed about ten days in terms of the little groin strain. Nikola played 70 minutes for the development group (earlier this week) and showed a great attitude in that game. I just wanted to give him a little time for the time he missed for the games."