Spencer signs for Cheltenham
Spencer impressed Allner sufficiently during a trial spell at Whaddon Road, scoring twice in the pre-season friendly against Forest Green Rovers.
Allner admitted the promising marksman's displays had merited handing Spencer a second bite at professional football, after previously playing for Bristol City.
"Damien is young and has a lot of the attributes we are looking for," Allner told the club's official website.
"He has pace, strength, ability and can score a goal. He could be quite an exciting prospect for us.
"He will probably need a bit of time to settle in and will require plenty of encouragement because he has just come out of a bit of a downturn in his career.
"But he is only 20 and is definitely worth giving the opportunity to.
"When Julian Alsop is not in the side we perhaps lack a bit of physical presence up front and hopefully Damien will be able to provide that for us as well."
Spencer, himself, revealed he had received other offers but was swayed by his first impressions of the Second Division newcomers.
He said: "I had the chance to go to Wycombe on trial and had offers from a couple of non-league clubs, but when I got a call from Mike Davis and came down to Cheltenham I liked what I saw.