The defender was knocked out for around five minutes in an accidental collision.
He went into a challenge with Zak Sharp during yesterday’s 5-0 victory, and the latter’s knee caught the US centre half on the temple.
Following treatment from US physio Stefan Stefan and the Baffins physios, an ambulance was called and Birmingham taken to hospital.
‘We thought he might be out for two weeks, but the NHS protocols now say it’s three weeks for a head injury,’ said US boss Glenn Turnbull.
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‘It’s a shame as Harry had worked really hard during the summer.
‘He was out cold for a while, but I knew he was ok when he came round and told me ‘look what happens when I stay in on a Friday night instead of going to the pub!’
‘The three physios were exemplary, just fantastic, they worked together as a team to treat Harry.’
Birmingham joins midfielder Harry Bedford - who suffered a knee injury in the first training session - on the sidelines.
He misses Tuesday’s friendly at Horndean - managed by his dad, Michael - and further games against Hereford-based Westfields, Cowes and Littehampton.
US’ Wessex League opener is scheduled for September 5 at Laverstock & Ford.
Following their shock 5-0 caning of Baffins, US have now belted 11 goals in two friendlies.
Dec Seiden gave them a first minute lead in a game played at Moneyfields, picking himself up after being fouled by Sharp to score from the spot.
Into the second half and skipper Tom Jeffes doubled the lead before Dan Sibley netted either side of a James Franklyn effort.
‘It was a pleasing result,’ said Turnbull. ‘I know you don’t win anything in pre-season, but it was good to get minutes into legs and hopefully it’s a good indication.
‘If anybody had watched the game they wouldn’t have said we were the Wessex Division 1 side and they were the Premier team.’
US used 21 players and gave a first team debut to teenage newcomer Vinny McGhee, who had played for the reserves in midweek. Goalkeeper Tom Price was the only one to complete the 90 minutes.