The 19-year-old has linked up with the Pilgrims for the 2022-2023 National League North campaign – although he is still eligible to feature for the Imps in EFL Trophy action.
Long is highly-rated by his parent club and has previously spent time on loan at Gainsborough Trinity and Irish side Drogheda United.
His played 45 minutes of Wednesday evening’s pre-season friendly against Notts County.
Reflecting on Long's arrival, manager Paul Cox said: "He did well with a very competent first half performance.
"I am glad that we have got some competition now with him and Rhys.
"I want competition all the way through the squad and I now have food for thought."