United remain in negotiations with Sporting for Fernandes though are yet to settle on an agreed transfer fee, with the Primeira Liga club demanding €80m (£68m).
Super-agent Mendes is involved in the Fernandes negotiations, which have seen Sporting propose that United pay €65m up front with a further €15m in add-ons.
United are reluctant to spend that much and value Fernandes closer to an initial €50m (£42m), given that Tottenham Hotspur touted Fernandes as a €45m prospect last summer.
A compromise remains possible before the close of the winter transfer window on 31 January but talks will break down if one is not found.
When asked on Wednesday whether Fernandes would join United this month, Mendes told BBC Sport that he was unsure.
“If he doesn’t leave now, he will leave for sure in the summer because Sporting have already spoken with other clubs,” Mendes added.
“Something will happen but I am not sure if now or at the end of the season.”
Little is understood to have changed in negotiations for Fernandes since United knocked back Sporting’s €80m asking price at the weekend, though talks continue.
Mendes was attending a meeting of the Association of Football Agents on Wednesday in north London, with fellow agents Mino Raiola and Jonathan Barnett in attendance.
Many of world football’s top agents are hoping to fight plans by world governing body Fifa to limit the amount intermediaries can earn from facilitating transfers.
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