But Heffernan looks certain to start in the SPL against Hibs tonight after hitting six goals in his last five games.
Shiels has been assessing Boyd's sharpness in training after the SPL's all-time top goalscorer re-signed for the club on Saturday, while Sheridan was given a late run-out in Saturday's 1-1 draw with Aberdeen after recovering from a hamstring injury.
Young forwards Rory McKenzie and William Gros have also featured during Kilmarnock's recent five-match unbeaten run and Shiels knows he faces a difficult task in keeping all of his forwards happy.
"It's going to be hard to do," he said. "It's just about playing for the shirt now. It's hard to get in with Heffernan doing so well, so it will be hard for Cillian to get straight back in."
Boyd scored 67 goals in his first spell with Killie and the 29-year-old remains top SPL scorer with 164 after returning from spells in England, Turkey and the USA.
"I think we will get a lift from him and he is one who will certainly add something to our team," Shiels said. "It's going to be a bit soon for him. He'll need a bit more training and game-specific preparation."
Hibs, who sit a point and one place above Kilmarnock in sixth, visit Rugby Park on Sunday in the Scottish Cup quarter-finals, but Shiels is not formulating a plan over two matches.
He said: "We take the games as they come. We try to win the first game then we'll look at the second game."