Why Sheffield United's new boys will be 'involved' at Reading ahead of possible debuts in FA Cup clash
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The duo both signed on loan at Bramall Lane until the end of the current season, although United have the option to make their deals permanent in the summer.
And Greek international Retsos could be handed a United debut at right centre-half if Chris Basham is rested, while speedy striker Zivkovic may make a cameo appearance off the bench.
"The new boys will be involved," Wilder confirmed.
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Hide Ad"They've been pushing hard but the form of Chris Basham has been outstanding, and we have really good options at the top of the pitch.
"The skipper [Billy Sharp] is taking his chance with his all-round play, and he can finish too, while David McGoldrick is coming back, we have Lys Mousset... I have to make decisions, looking from Monday to Friday.
"It's easy sometimes to ask 'why did he make that decision' but the game is sometimes better suited to a particular individual.
"It's important, looking in the week leading up to a game, who I think can make the difference.
"They're all giving me a headache, which I have to embrace and deal with."
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