Striker Steven Thompson's chances of making the squad for Saturday's SPL trip to Perth remain in the balance.
The Scotland Under 21 cap has never fully shrugged off the ankle-knock he picked up during United's pre-season trip to the Netherlands, which restricted his match action on the tour to just seven minutes.
Although Tommo has been a regular visitor to club physio's treatment room, it's doubtful that he can prove his fitness in time for the weekend clash with St Johnstone.
Boss Alex Smith said," Tommo will continue to receive treatment and we won't have him on the training pitch until Friday. "His ankle is a bit puffy and at this stage it's just a case of wait and see - it's a shame for the lad as he's obviously been keen to get the chance to impress this season."
Newboy Stephen Carson is another McDiarmid Park doubt, while Jamie Fullarton has already been ruled out.
Several first team squad members, including the likes of David McCracken, Hasney Aljofree, Hugh Davidson, and Alan Combe took part in a midweek closed-door affair at Livingston.
"These kind of games are always worthwhile, as it keeps the match fitness up and allows us to have a look at a few things," added Smith.