Aston Villa manager Roberto Di Matteo
The new Villa boss is expected to overhaul his squad following the club's relegation to the Championship, and winger Sinclair is attracting major interest.
The 27-year-old forward joined Villa on loan from Manchester City back in January 2015, and earned a permanent switch to Villa Park last summer having scored three goals in 12 games during his loan spell.
Last term, Sinclair was in and out of the Villa side, but did notch six goals in 27 games, prompting interest from one of his former managers.
Celtic manager Brendan Rodgers celebrates with Nadir Ciftci after beating Lincoln Red Imps
Celtic manager Brendan Rodgers worked with Sinclair at both Chelsea and Swansea City, guiding him to 35 goals in 90 games during their time together with Swansea.
Now, with Rodgers looking for additions following his side's 3-0 win over Lincoln Red Imps on Wednesday evening, Sinclair is a top target as the new Celtic boss pursues a reunion with the wide man - but it's proving to be difficult for the Bhoys to land their man.
The Daily Mail report that Celtic have had three bids rejected for winger Sinclair, with the latter of which totalling £2.2m, which was rejected earlier this week as Villa allegedly want close to £6m.
It's also added that Villa boss Di Matteo has spoken to Celtic manager Rodgers about the situation, and has warned him that Villa are in no rush to sell Sinclair despite Celtic's desire to get a deal agreed by the end of today.
If Celtic are to beat the Champions League registration deadline ahead of a clash with Astana, they will have to considerably increase their offer for Sinclair, given Di Matteo's warning to Rodgers.