The 20-year-old picked up the injury in training on Friday and is doubtful for the upcoming games with Bayer Leverkusen and Chelsea at St James’ Park.
"We were doing some docile shooting practice on Friday and he felt a little tear in his Achilles," Sir Bobby confirmed.
"Later in the afternoon, he was not feeling good and we ruled him out this morning.
"He will have a scan and I don't know how serious it is."