Tkachuk signed a two-year deal with the Blues on Saturday, less than a week after re-acquiring the bruising forward in a trade with the Atlanta Thrashers.
With Tkachuk set to become an unrestricted free agent Sunday, the Blues had to act fact or risk losing the 35-year-old to the open market.
Terms of the deal were not released, though president John Davidson said Tkachuk agreed to take less money in the first year of the contract to give the Blues more financial flexibility in the coming off-season. Tkachuk made US$3.8 million last season.
Davidson said acquiring Tkachuk's rights did pose a risk, but it was one his team was willing to make.
"If we were able to get deal done, great," Davidson said in a conference call from Kansas City, Mo. "If we could not put a deal together, we knew come July 1st what we had to try and accomplish.
"Now, with a part in place, and a major part, we can concentrate on other parts of what we're trying to do."
Tkachuk, a two-time all-star, was dealt to the Thrashers on Feb. 25 for Glen Metropolit, first-and third-round draft picks in 2007 and a second-round pick in 2008. The six-foot-two, 225-pounder finished with seven goals and eight assists in 18 games in Atlanta, helping the Thrashers reach the Eastern Conference playoffs for the first time in franchise history.
Atlanta promptly lost its first-round series in four games to the New York Rangers.
Tkachuk, who finished with 27 goals and 31 assists in 79 games between the Blues and Thrashers, said he was caught completely off-guard by Tuesday's trade.
"I never saw it coming," said Tkachuk. "I was excited, but very confused."
Blues head coach Andy Murray said Tkachuk would have commanded a ransom on the free market.
"Keith going on the market (Sunday) would be a highly-valued player," said Murray. "There's no question he could have made more money, but Keith wanted to be a Blue."
Davidson, Murray and general manager Larry Pleau spent the week talking to Tkachuk about the merits of returning to St. Louis which went 34-35-13 last season and missed the playoffs for the second straight year. Tkachuk wouldn't share what the two sides talked about, but he said money played no role in his decision to return.
"You don't just talk contract," said Tkachuk. "You have to get a feel for each other. This is a critical time in my career. Who knows how much longer am I going to play?
"My family loves it in St. Louis so it was definitely a priority for me to come back here. I just didn't think it would happen the way it did."
The Blues are expected to be major players in the summer's free-agent bidding after having one of the lowest payrolls at the start of last season.
"We've had blessings from our ownership to do what we think is right for the Blues," said Davidson. "Ownership has been very, very good with us regarding our future."
Tkachuk is a believer.
"I see a lot of good things happening," said Tkachuk. "They're going to be very active in making this a better hockey team.
"I haven't won yet. I want to win. I'm very confident the Blues will make a statement next year."
Tkachuk has 473 goals and 453 assists in 976 career games with Winnipeg/Phoenix, St. Louis and Atlanta. It's possible he could reach the 500-goal, 500-assist and 1,000-game plateau all next season.
"We wanted him to have those milestones in a Blue uniform," said Davidson.