Sinclair, who will officially sign on 1 July, scored just once in his 16 Blue Square Premier appearances following his arrival on loan from Kenilworth Road in February.
"If anything, he thinks that was the thing that let him down while he was here," said Holmes. "He missed a few chances but hopefully he can put that right next season."
Holmes also reveals that he is keeping an eye on another former loan signing from Luton, Ed Asafu-Adjaye.
"We're just keeping our ears open as to what's going on," said Holmes. "We do know that Luton have offered him a contract."
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