The Red Devils were not expected to carry out any business today but BetStars reckon the 25-year-old is almost a certainty to make a switch.
And United could still make late moves for Inter Milan ace Ivan Perisic and Danny Rose as well.
BetStars Director of Trading Ian Marmion said: “As the transfer window edges closer to its deadline, Manchester United could continue their summer splurge by adding a few more big-name signings to their ranks before the clock strikes 11pm.
“Sergi Roberto’s versatility to operate as a midfielder and defender has caught the interest of Jose Mourinho and the Barcelona player is priced at 17/20 to complete a move to Old Trafford today.
“United have also been heavily linked with Internazionale’s Ivan Perisic throughout the summer window and the Red Devils are valued at 5/1 to make a late dash for the Croatian winger.
“Danny Rose has been hovering near the Spurs exit door this summer and is priced at 13/5 to become the latest Tottenham wingback to make a move to Manchester before the end of the window.”
United have has a busy summer so far, bringing Romelu Lukaku, Nemanja Matic and Victor Lindelof to Old Trafford.
Jose Mourinho has also re-signed Zlatan Ibrahimovic as he continues to recover from a knee injury.
United currently sit top of the league after winning their first three Premier League matches of the season.
After the international break they return to action against Stoke.